# prepping old wood for painting newb questions



## Nestor_Kelebay (Jun 17, 2008)

afabco said:


> Got some old wood from some tearouts that's still in pretty good shape. Some of the pieces still have some old paint, no idea what base(s) on them here and there. Since what I've kept is still very usable so I'm building some frames and whatnot. I want to paint them, more from a preservation and maintenance view than to make it pretty. I'm a newbie to this painting stuff, so after reviewing webstuff and some reading, I want to see if I understand what needs to be done to paint these pieces: 1. knock off all the loose stuff. 2. Rough sand each piece 3. Using rags and a brush, wipe down each piece with mineral spirits. 4. Wait a day. 5. Using rags and a different brush, wash down each piece with a TSP solution. 5a? Do I need to do a water wash or something after this to remove any residues from the TSP wash? 7. Put the frame together, glue and screws and whatnot. 8. One primer coat of exterior grade oil or latex based primer. 9. Wait a day. 10. Two coats of exterior grade oil or latex based paint. Does this sound right? Thanks!


 
The washing with mineral spirits will remove any residual oil or grease from the wood if there is any. Mineral spirits will evaporate from the wood completely without leaving any residue. This is the purpose in waiting a day after cleaning with mineral spirits... for the mineral spirits to evaporate. You might want to wait two days.

I see absolutely no purpose in cleaning the wood with TSP. TSP was commonly used prior to the 1970's to clean walls prior to painting them. The reason for cleaning with TSP was because it etched the gloss of oil based paint, making it rougher so that the next coat of paint would stick better. If you have an oil based paint on your wood, then I'd clean them with TSP if you intend to repaint that wood. But, there is no point whatever in cleaning bare wood with TSP, or latex painted wood with TSP.

If you presume I don't know what I'm talking about and you do wash with TSP anyhow, then I would also clean the residual TSP off the wood with a clear water wash.

If you sanded any paint on the wood down in Step #2, you really don't need to prime as well. A top coat of paint will stick to sanded paint just as well as it'll stick to a primer. Priming over sanded paint won't do any harm, tho.

If you do prime bare wood, try to use an oil based primer. Alkyd resins are MUCH smaller than latex resins, so they penetrate deeper into the wood for better adhesion. Exterior alkyd primers will dry to a much softer film than interior alkyd primers, so if you intend that these frames will spend their lives outdoors, use an exterior alkyd primer. If they'll be spending their lives indoors, use an interior alkyd primer. There is NO difference between an interior and exterior latex primer. Latex primers can be used indoors or out.

I much prefer exterior alkyd paints over wood outdoors than exterior latex paints, so I'm biased. Exterior alkyd paints dry to a harder film than latex paints, which stands up better to weathering. But, no one is going to take away your DIY'er merit badge if you use an exterior latex paint on wood outdoors.


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## afabco (Aug 2, 2008)

Thank you for the reply. This is wood left over from some old cabinets and shelves in the garage of the 1950's house that owns me now. It'll mostly be used for functional stuff. This particular one is a frame to hang my bicycle from. Another is a rack to put my offseason tires onto. So 'pretty' is not really a main concern, but it would bother the OCD in me that all the various pieces of leftover paint (multiple colors, sometimse two layers worth) are here and here. Plus I'm expecting the paint to help protect against the wood warping, splitting and deterioting, and doing whatever else unfinished wood does in a garage. I have no clue what the various paints are...oil? water? latex? old cat gut?...who knows? I did give a quick rough sanding on all surfaces (painted and otherwise) just to rough things up a bit, knock some of the loose stuff off. I did the mineral spirits thing this afternoon. So I'm assuming that the TSP wash is not required at all? or just over the old paint on the pieces that still have some old paint on them? And as far as a clear water wash, will standing each piece on end and spraying them down with a waterhose do it? Of course then willl probably have to wait until next weekend to tbe sure the wood is fully dry. Then probably painting with some sort of exterior alkyd (the frames and racks aren't outside, but they are in a garage/shop area). This sound like a plan? Thanks!


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## sirwired (Jun 22, 2007)

Nestor, wouldn't the flexible film of an exterior latex be an advantage over alkyd on wood, which expands and contracts with the weather?

SirWired


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## Guest (Aug 3, 2008)

Saying that there is no difference in interior or exterior latex primers is very misleading. There are several differences. Use what is made for the application at hand. All the primers on my shelves use different resins between the interior and exterior versions. As for paint, I agree that oil is the tougher finish. I'd use oil if you are making furniture or anything that will have wear on it like people sitting or standing. And I don't agree that Oil stands up better to weathering. Acrylics remain flexible and do not get brittle like an oil finish so they don't crack as easily. They also hold their color better and don't chalk as quickly. Either would protect the substrate fine though.


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## Nestor_Kelebay (Jun 17, 2008)

afabco said:


> Plus I'm expecting the paint to help protect against the wood warping, splitting and deterioting, and doing whatever else unfinished wood does in a garage.


Afabco:

Your wood won't deteriorate at all inside your garage. Wood deteriorates rapidly outside because it absorbs moisture (mostly rapidly through it's end grain) from the outside air and any rain that falls on it and then dries very rapidly from the Sun and the wind. The absorbtion of moisture into the wood, subsequent diffusion of moisture through the wood, and then rapid drying (again, most rapidly at the end grain) is what causes wood outdoors to split at it's end grain. The absorbed moisture causes the wood cell WALLS to become thicker (and softer), and that causes the wood to swell as it absorbs moisture. The subsequent rapid drying (most rapidly at the end grain) causes the wood to want to shrink again, and more rapid shrinking at the end grain than in the middle is what makes the wood split at the end grain. Also, it's exposure to the Sun that causes the wood cell walls at the surface to deteriorate and turn gray and the "lignin" that holds the wood cells together to deteriorate, allowing the wood cells to separate from the rest of the wood.

None of those conditions apply INSIDE a garage. Your wood will absorb some moisture from the air outdoors, but the drying in the shade won't be nearly as rapid. Also, there won't be any direct sunlight on the wood, so the deterioration from UV light (greying and cells coming loose from the rest of the wood) won't occur inside a garage.

The primary difference between interior and exterior ALKYD primers and paints is that exterior alkyd primers and paints dry to a softer film that is elastic enough to stretch and shrink with wood outdoors. Latex paints are way more elastic than you need to stretch and shrink with the wood, and so the primary difference between interior and exterior LATEX paints is that exterior latex paints have much more UV blockers and mildewcides added to them.

For your applications, I really don't see that you need an exterior paint. I would be inclined to just use an interior paint for these racks and frames inside your garage.

Do you have any wood stored in the rafters of that garage? If so, inspect it. Are the ends of the boards split from swelling and shrinking? Prolly not. Is the wood grey and fuzzy? Again, Prolly not. For that matter, look at the rafters themselves. If they don't look deteriorated (split at ends and grey fuzzy surface) then they are experiencing neither swelling and shrinkage, nor UV exposure. And, if there's no mildew growth on that wood, then you don't really need an exterior paint to protect that wood. You would do better with a harder and more protective coating over it to keep it from getting scratched up from the bikes and tires. In that case, I'd be inclined to use an interior alkyd primer followed by an interior alkyd paint.



> So I'm assuming that the TSP wash is not required at all? or just over the old paint on the pieces that still have some old paint on them?


If you have the same kinds of paints on various pieces of wood, try cleaning each KIND of paint on one piece of wood only with a strong solution of TSP. If you can them see a lower gloss on any paint that was cleaned with TSP, then that was oil based paint, and cleaning it with TSP will "etch" the gloss of that paint so that any subsequent coat of paint sticks better. If you see no difference in the glossiness of the paint after cleaning with TSP, then cleaning with TPS is a waste of time. It could be that the oil based paint was already a flat paint, or it could be that you're cleaning latex paint with it.



> And as far as a clear water wash, will standing each piece on end and spraying them down with a waterhose do it? Of course then willl probably have to wait until next weekend to tbe sure the wood is fully dry.


I've never done it, but I'd say that's all you need to do. TSP is soluble in water, so you don't need to scrub it to get it off the wood.



> Probably painting with some sort of exterior alkyd (the frames and racks aren't outside, but they are in a garage/shop area). This sound like a plan? Thanks!


Take a look at any wood that's been stored in your garage for a long time, even any exposed studs and rafters. If you don't see splitting of the wood at it's end grains and the surface deteriorating to a grey fuzz, then I'd use an INTERIOR alkyd primer and paint on your racks and frames.


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## Nestor_Kelebay (Jun 17, 2008)

SirWired:

Latex primers and paints are more than elastic enough to stretch and shrink with wood outdoors. They don't need to be as elastic as they are to keep up with wood expansion and shrinkage.

You can, in fact, use a harder latex paint on wood outdoors. I've never tried it because I don't have a wood porch, but you can use a latex "Porch & Floor Enamel" on an outdoor wood porch. Latex Porch & Floor paints will use a cross linking acrylic resin, which is a resin that forms a film through coalescence just like normal latex paints, and then over the following months forms crosslinks between the latex resins making for a stronger and harder film. My understanding is that these latex Porch & Floor paints don't form as hard and strong a film as even an exterior alkyd paint.

Interior alkyd paints simply aren't elastic enough for wood outdoors. I know they make exterior alkyd paints by using different alkyd resins that form softer films, but I don't know precisely why those films are softer. I don't know if it's because exterior alkyd resins have FEWER (?) unsaturated sites (and would therefore take longer to dry to a solid film) or if exterior alkyd resins have much longer fatty acids in them (that would make for larger resins that would stretch further when pulled on) or what exactly the difference(s) are. I do know that the reason interior alkyd paints don't last long on wood outdoors is that the films they form are too hard and rigid and just aren't elastic enough to stretch and shrink as far as the wood will, and the result is that both the paint film cracks (in some places) and the paint lets go of the wood (in other places) and the result is that the paint film is seen to both crack up and peel off.

Normally, you WANT a paint to form a hard and strong film, but in the case of alkyd paints, the films they form are so rigid that they can't accomodate the swelling and shrinkage of the wood, so they have to modify alkyd resins so that they form softer and more elastic films before they last well on wood outdoors.

Latex paints, on the other hand, stretch like a bungee cord. They're softer and more elastic than they need to be for wood.


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## Nestor_Kelebay (Jun 17, 2008)

Poppameth:

You said: "Saying that there is no difference in interior or exterior latex primers is very misleading. There are several differences. Use what is made for the application at hand. All the primers on my shelves use different resins between the interior and exterior versions."

When I read your response, I figured "Where is there any room for any difference?" Every latex is elastic enough to stretch and shrink with wood, and you don't need to add UV blockers and mildewcides to a primer because it'll be covered with paint anyway. So, where could any difference lie? Clue: Poppameth said it was in the resins somewhere.

(For those who want to know why a "low-VOC" primer or paint would require a different resin than a regular VOC primer or paint, see the PS at the bottom of this post.)

So, I did a Google search for "Interior latex primer" and "Exterior latex primer" and I found two web sites that, to my mind at least, explain the difference, and also indicate that there is very little difference at all.

Take a look at this one:

http://www.benjaminmoore.com/bmpsweb/portals/bmps.portal?_nfpb=true&_windowLabel=contentrenderer_1_10&contentrenderer_1_10_actionOverride=%2Fbm%2Fcms%2FproductSearch%2FgetProductDetails&contentrenderer_1_10NodeUUID=%2FBEA+Repository%2F36008&_pageLabel=fc_productsspecs

It's an "interior" latex primer that is low odor because it has low VOC's. That is, the reason they recommend it for use indoors is because of it's low odor.

And take a look at this one:

http://wallsalive.com/pdf/specs/Spec C3000.pdf

which is an "exterior" latex primer that has 124 grams/liter of VOC's in it.

That is, I expect the difference you speak of is that latex primers that specify that they are "interior latex primers" will be low odor whereas primers that specify that they are "exterior latex primers" won't be of the low VOC, low odor variety, and using them outdoors isn't a problem because the odor won't be noticed as it would indoors.

But, from what I can see, either can be used indoors or out. That don't mean I'm seeing everything tho. And so I can't be 100 percent certain that the only difference between interior and exterior latex primers is the olafactory one.

My understanding is that you own/manage a paint store. Can you check with your suppliers or manufacturer's sales reps to find out what exactly the difference is between "interior" and "exterior" latex primers?

We all learn from each other in these forums and maybe this will be my turn.

The following is just for people who don't understand why the solvents used in primers and paints have to be matched to the resins in the primer and paint.

PS: Why do we need a difference in the resins if it's a low-VOC primer?
Latex paints form films by a process called "coalescence". Latex paint consists of a slurry of tiny hard clear plastic particles (called resins) and coloured pigment particles suspended in solution of water and a water soluble solvent called a "coalescing agent". When the paint is applied to the walls, the water evaporates first, resulting in the plastic resins being surrounded by the coalescing solvent at an ever increasing concentration. Those solvents soften the plastic particles enough to make them both stick to each other and pull on each other (due to surface tension and capillary effects) so that those plastic resins form a soft continuous film of plastic. Then, the coalescing solvents evaporate from the paint film, creating that "newly painted smell" in the room.
It is these coalescing solvents that are the primary source of "VOC"s in latex primers and paints. (The glycerine they add to the latex paint when tinting it and the propylene glycol they add to it as an antifreeze so the paint won't freeze as readily also add to the VOC's that evaporate from the paint film, too, but the quantities of these in the paint are less than coalescing solvents.) So, if you have to make a low-VOC paint to meet government regulations, you either have to use a different resin that will coalesce at a lower coalescing solvent concentration, or you have to use a coalescing solvent that will react chemically with the paint and therefore actually become part of the paint film rather than evaporate into the air.


Follow up:
After posting the above, I thought to myself: why not do a Google search for "exterior latex primer" AND "low odor". My thinking was, if the only difference is the odor of the solvents as they evaporate from the drying primer, I should have NO hits because no one would make a low odor primer and call it an exterior primer, right?
That would tell me whether the difference is entirely in the odor or not.

Anyhow, I came across this web site:

http://www.cloverdalepaint.com/info/pdf_tds_ar/10-0125.pdf

Which talks about their "Horizon Exterior Latex Primer" as follows:

Intended Uses:
Horizon Exterior Latex Primer is ideally suited for use wherever paint odor is a problem such as schools, hospitals, nursing homes, offices and other smell-sensitive areas. This primer has been designed for use with HORIZON topcoats providing a complete low odor and environmentally friendly system. Suitable for
brush, roller or spray application on exterior and interior substrates.​ 
Which is confusing because this Cloverdale Paint Company seems to be saying that the important difference between this primer and others is it's low odor, and specifies that it is therefore well suited for INTERIOR applications such as schools and hospitals where odor may be a problem for some sensitive people.​ 
That is, they're calling it an "exterior latex primer", but they're recommending it for interior use because of it's low odor.

My web search so far seems to suggest that the only difference is the odor.

Benjamin Moore says this about it's ECO SPEC Interior Latex Primer Sealer 231:​*General Description​*A low odor, low VOC, 100% acrylic interior latex primer sealer with spatter resistant properties.
Ideally suited for commercial, facility management, and residential applications. ECO SPEC​​® Interior Latex Primer Sealer (231) does not have the odor of conventional primers which contain ingredients known as Volatile Organic Compounds (VOCs). Always use ECO SPEC® Interior Latex Primer Sealer (231) as a first coat when a low odor, VOC free primer/finish system is required.

Still, I'd be curious to find out what the paint company sales reps say is the difference.​


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## Guest (Aug 4, 2008)

Chances are the reps don't have a clue. I'll have to question the tech department on it. It's been my understanding in the past that the exterior wood primers are an acrylic/vinyl blend with the vinyl being of a different tougher grade than the interior vinyls. They are also designed with a slightly different molecular structure to allow the primer to penetrate deeper. It's also taken longer for exterior latex primers to dry for a couple of reasons, 1) to allow greater penetration and 2) to allow more open time in more direct sunlight than you have to deal with inside. Of course a lot of that has changed with all the 100% Acrylic indoor/outdoor primers on the market now.


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## Nestor_Kelebay (Jun 17, 2008)

Penetration into wood is largely determined by the size of the resin. The old linseed oil based primers penetrated best because they were quite literally the size of plant oil molecules. Alkyds are "clumps" of plant oil fatty acids, so they're very much bigger. Latex resins are absolutely huge compared to even alkyd resins, so there's NOTHING they can do to make a latex resin penetrate deeper into wood. It's like trying to get a camel to pass through the eye of a needle.

If someone does tell you that exterior latex primers penetrate better into wood than interior latex primers, then I would ask him to provide some reliable reference from his company to substantiate that claim, or at least explain how they're managing to get a little more of the camel through that needle.

It really doesn't make sense to mix PVA and PMMA resins when making paint because it simply complicates things without providing any benefit. There are a great number of PVA and PMMA resins available at different prices and with different characteristics, but generally well defined traits in each kind of plastic. Mixing resins is only going to complicate matters because then you have to ensure that the additives and coalescents you use work properly with both resins. And, there is no real benefit in doing this. PMMA resins are better at everything than PVA resins except being inexpensive to buy. If a person wants to cut corners on the quality and cost of the paint they're making, it's easier just to buy a less expensive PMMA resins, or a less expensive additives package. Mixing binders just adds one more variable that both increases costs (cuz now you have to store two resins instead of one) and increases potential problems (cuz now your workers might screw up on the ratio of resins to mix).

The better way to make a paint is to focus on the requirements you need (like hardness, water resistance, UV resistance, blocking, alkali resistance, etc.) and then choose the resin that's best suited to those specific requirements. If you want to lower the cost, you can buy a less expensive resin that isn't quite as good from a different company instead. But, truth is, that most people are willing to pay more for the best paint because they realize the labour associated with both is going to be the same anyway, so it make sense to invest their labour into the best paint to get the best overall product at the end of the day. It's kinda like a sculptor choosing a lesser quality marble for his statue. What sculptor would do that?


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## afabco (Aug 2, 2008)

Many thanks for all the replies and discussion. I think I've got a pretty good idea of the sequence now.

I still don't have a handle on the 'alkyd' paint though. 

I was in lowes yesterday (for some other stuff), and just took a wander through the paint section. I did not see anything with the word 'alkyd' in it. What do I need to to be looking for?

Thanks again for all the assistance.


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## Guest (Aug 4, 2008)

That's because Lowes is trying to get away from selling oil at all. You should go to a real paint store for this. 
I just talked to the Richards Paint rep today. Evidently they are of a like mind with you Nestor. They are changing of their primers to an indoor/outdoor formulation because they felt that the differences were not substantial enough to warrant having separate products.


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## Nestor_Kelebay (Jun 17, 2008)

Poppameth:

Did you question the sales rep on whether the difference between their former "Interior" and "Exterior" latex primers was strictly whether or not they were low odor?

Afabco:
Just ask for an "oil based" primer or paint at any paint store, and they'll give you an alkyd primer or paint. Unlike latex primers, there is a big difference in interior and exterior alkyd primers and interior and exterior alkyd paints.


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## Guest (Aug 5, 2008)

I didn't have time to question him in depth on it unfortunately. It was too busy in the store today. He did tell me that at least for them it didn't have much at all to do with odor issues.


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## Nestor_Kelebay (Jun 17, 2008)

You said it was "Richard's Paint".

I Googled Richard's Paint and found out their toll free number is 1-800-432-0983 in Florida.

I'll give them a call, hopefully tomorrow. I want to hammer this thing out for myself. It just doesn't make any sense to me that they're modifying the binder to make an "interior" versus "exterior" latex primer. There would be no reason to do that, and if this company makes paint, they should know that.


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## Guest (Aug 6, 2008)

I believe Tom Koby is the guy you generally want to talk to at Richards. If he doesn't know an answer he'll find a chemist that does.


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## Nestor_Kelebay (Jun 17, 2008)

Thanks, Poppameth.

I phoned Tom today and he referred me to Al Guzman who's the head chemist at Richard's Paints.

Al told me that they do make separate indoor and outdoor primers, but that they make over 200 different formulations and to tell me specifics on any of their products he'd have to know what the product numbers were. He says one of the differences between interior and exterior latex primers is that the outdoor primers they make will all have mildewcides added to them because the primer may not be topcoated right away. He said that if the primer gets left unpainted, then the mildewcide in the primer will prevent mildew from growing on it. He also told me that they may also use a 100% acrylic binder in some of their exterior primers for exactly the same reason; so that it'll stand up better to UV light from the Sun if it's not top coated with paint.

When I asked if another difference could be whether or not they're low odor, he said that could be another difference too. He said he'd have to know the product number before he could discuss specific differences.

I was surprised that they used mildewcide in their primer and that they'd use a 100& acrylic binder IN CASE the primer wasn't painted. 

I didn't want to say this to Richard's Paint, but it seems to me that if they're charging more for their exterior latex primer because it has mildewcide and a more UV resistant binder in it (just in case the primer is left unpainted), then 99 percent of the time the extra money spent by the customer is wasted. That's because 99 percent of the time, the primer IS top coated with paint right away. Once there's a coat of paint on top of the primer, it's the paint that has to deal with the UV rays from the Sun and the mildew spores in the air.

If you could get me the product numbers for the interior and exterior latex primers, I wouldn't mind asking Tom Guzman if someone is planning on painting over Richard's exterior latex primer right away, whether they would still be better off buying their exterior latex primer or saving some money and just using Richard's interior latex primer on the outside of their house.

I think that's a darn valid question considering that they're telling me that they make their exterior primer with mildewcide and a more UV resistant binder IN CASE the primer isn't painted over. (?)


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## afabco (Aug 2, 2008)

Once again, thanks to all. The oil <-> alkyd link was what I was missing.

now for the weekend activities....heh.

enjoy!


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## Guest (Aug 6, 2008)

I stock Richards primers almost exclusively because they are very comparable to Zinsser and cost me far LESS than most other primers I can buy. For example, Richards 7-44 100% Acrylic is an indoor/outdoor very much like 1-2-3 (which also has a mildewcide) except that it's a dead flat primer instead of having a sheen, and I retail it at $26.95 / gal. which around here is very good unless you go to Lowes for Zinsser. 

Lets see:
7-11 is an interior latex primer/sealer/undercoater
7-22 is an int/ext alkyd enamel primer/sealer/undercoater (fast dry)
7-33 is an int/ext alkyd enamel stain killer (fast dry)
7-55 is a synthetic odorless alkyd stainkiller (fast dry)
Those are the main ones I carry right now.
On their distributer list:
6-210 is an Int/Ext Modified Acrylic House Primer
SR-933 is their long oil Alkyd house primer

Edit: Full product list http://richardspaint.com/allproducts.cfm

That's all I've got on my list other than industrial products. I know they make far more than that though. They private label a lot of things that they'll sell me on request if I need it.

I've been trying to get a hold of my Pratt & Lambert rep but he's a pain to deal with. Here is the list of their primers. http://prattandlambert.com/products/homeowner/primers/interior/


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## Nestor_Kelebay (Jun 17, 2008)

Poppameth:

That explanation by Al Guzman that they use mildewcide and a more UV resistant binder in their exterior primers IN CASE the customer doesn't paint over the primer didn't make any sense to me because it would mean that the 99% of the people buying Richard's exterior latex primer and painting over it are wasting money on the extra mildew and UV resistance in the primer. I thought I would get a better (or at least more plausible explanation) from Pratt & Lambert. 

I phoned Ryan Kearney at technical support for Pratt & Lambert for the US at 1-800-289-7728. I asked him what the difference was between P&L Suprime Interior Latex Wall Primer and their Suprime Exterior Latex Wood Primer.

He says the only difference is that the exterior latex wood primer has more mildewcide in it. He says that dormant mildew spores are more common outdoors, and by painting over bare wood outdoors, there's a good chance that those mildew spores will become active and start to grow, with the result that the mildew could grow under the primer. Having more mildewcide in the primer ensures that the mildew spores are killed before they grow.

Ryan told me that without this mildewcide, mildew spores would feed on the latex primer, and would be visible under a top coat of latex paint.

With utmost respect, I would disagree with Ryan in regard to the last statement. Mildew spores, just like everything else, need both food AND WATER to survive. Aside from the question of whether or not a latex primer would be a food source for mildew, it seems to me that if there wasn't sufficient moisture in the wood for the mildew to grow before priming, the little bit of moisture that comes from the primer isn't going to last long enough to sustain any mildew growth for long.

That is, once the water evaporates from the primer and subsequent coat of paint, where does the water come from to support the continued growth of mildew if there was insufficient water to support the growth of mildew on the bare wood.

Anyhow, according to P&L technical support, there is no difference in binder used between interior and exterior latex primers, the only difference is that there's more mildewcide added to the exterior latex primer.

My feeling is that Richard's exterior primer has more mildewcide in it for the same reason, and Al Guzman may not have been aware of that reason, and believed it was there in case the primer wasn't painted. But, I think it's fair to say that any difference between interior and exterior latex primers revolves around the amount of mildewcide in the primer.

I also asked about the Supreme Interior Deeptone Primer, and that one is just where they don't fill the can up as much to allow room in the can for plenty of colorant for making tinted primers.

I can't say I'm happy with P&L explanation either. Mildew needs water to grow, and there's no reason to believe that the mildew would continue to grow under the primer once the primer and paint are dry. If anything, the mildew would have less water available to it under a coat of primer and paint than it would have previously had when the wood was exposed to the weather. If the previous amount wasn't sufficient to support growth, I have difficulty understanding how growth could occur after priming and painting.

Anyhow, that's what I found out. I guess it's up to everyone in here to decide whether it's worth it to use an "interior" primer indoors and an "exterior" primer outdoors, or just keep a can of "interior/exterior" primer for use both indoors or outdoors. But, no everyone knows where the differences lay.


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