# Latex over alkyd primer



## Wisco (Jun 18, 2008)

I think the answer is yes. Latex is a superior finish to oil for a topcoat. If oil is used as a primer the thought is wood will absorb the oil and kinda "condition" and seal it. If you have multi layers of build up though I wonder if oil even has a place. JMO


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## slickshift (Aug 25, 2005)

TVC15 said:


> I've read that it is not acceptable to paint alkyd over latex


Incorrect


TVC15 said:


> either latex or alkyd may be applied over alkyd primer.


Correct


TVC15 said:


> If latex is the chosen final product, is the surface then, in the future, only to be painted with latex products?


Incorrect...technically
For the most part, it's best
But if needed, an alkyd product can be used
For example, an alkyd stain sealer may be desirable to block water stains, then re-coated with latex


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## Nestor_Kelebay (Jun 17, 2008)

You can apply EITHER a latex or oil based paint over EITHER a latex or oil based primer.

However, with that said, it's never a good idea to put a hard film over a soft one because the result will be that the hard top coat will be prone to "chipping" off because the underlying soft film will break more easily. So, I'd avoid using a hard film (like an alkyd floor paint) over a soft film (like a latex primer) on a working surface (like a shelf or floor or counter top). I would sooner strip off the soft latex primer and use a harder interior alkyd primer before applying the alkyd floor paint.

I believe both latex and oil based paints have their uses.

Latex paints are easier to clean out of brushes and roller sleeves.

Oil based paints dry to a harder film that's simply more durable than latex paints do. Pigments don't "clump together" in oil based paints nearly as much as they do in latex paints, so oil based paints can be factory tinted to a much higher pigment content for better hide. Oil based paints use vastly smaller resins that penetrate deeper into wood for better adhesion. Oil based paints adhere better to smooth surfaces than latex paints. Oil based paints dry slower and therefore provide better self levelling so that brush strokes are less of a problem when painting with a brush. Film formation in oil based paints is less problematic than in latex paints. You can apply an oil based paint in a Canadian blizzard. The paint will take all winter to dry, but when the weather warms, it will dry normally. Can't say that about latex paints. Oil based paints can be tinted much more than latex paints because the glycerine added with the colourant won't prevent proper film formation in oil based paints like it can in latex paints, so you can add more colourant to an oil based paint without worry. Oil based paints help support the US economy more because they're made from soy bean oil, made from locally grown soy beans, whereas the PVA and PMMA plastics used in latex paints are made from imported crude oil.

What we're being led to believe is that latex paints have advanced to the point where they are equal to oil based paints. I'm sorry to say that I have use both an alkyd floor paint and a latex "Porch and Floor Enamel", and there's just no comparison. The alkyd floor paint stands up well to foot traffic and stays looking good a long time, whereas the latex "enamel" doesn't. It's simply too soft to stand up well on a floor. It marks up too easily because it is so soft.

I'm sure both would do an admirable job on a ceiling, but where you need a user friendly paint that will provide a hard durable film, you need an oil based product.

I'm Nestor Kelebay and I approved this message.


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## sirwired (Jun 22, 2007)

Just thought I would mention that some exterior Alkyd primers have a "Critical Recoat Time" in which they MUST be topcoated (usually with water-base paint), or there can be nasty issues down the road with adhesion. (Due to differing expansion qualities of the two coatings.) If there is a critical recoat time, it will be spelled out on the can.

SirWired


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## Nestor_Kelebay (Jun 17, 2008)

Dmax:

What's wrong with top coating over a FLAT alkyd paint with a latex paint? Why would you need an alkyd primer in that case?

And, if I were to sand down the smooth surface of a glossy oil based paint to make it rough, why would I then also need to prime over that rough alkyd paint with an alkyd primer before a latex paint would stick? Wouldn't the latex paint stick as well to roughened alkyd paint as it would to an alkyd primer?


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## Nestor_Kelebay (Jun 17, 2008)

Dmax:

The binder resins used for primers generally are formulated to stick to surfaces better than those used for paints. In primers, the adhesion of the primer to the substrate is of primary importance, whereas paints have to stick well, but also have to meet other often more important requirements, like drying to a hard enough film that they stand up well on a floor or shelf.

The way primers improve adhesion of a paint is not that they grow teeth. Primers stick to their substrate well because the binder resin in them was chosen because it has excellent adhesion to the substrate the primer is intended to be used on.

The way primers promote good adhesion of the top coat to the primer is by the addition of "extender" pigments" to the primer.

Extender pigments are huge rocks almost large enough to see with the naked eye. A "micron" is 1/1000 of a millimeter or one millionth of a meter. A human hair is typically about 100 microns in diameter. The limit of human vision is about 20 microns. Anything individual thing smaller than that is simply too small for us to see. A red blood cells is about 5 microns in diameter (which is why blood looks like a red liquid, not a clear liquid with tiny red blobs in it). A latex paint "polymer colloid" or "resin" is about 0.1 microns in diameter and the smallest pigment used to make paint is the black pigment "soot" at approximately 0.001 microns in diameter.

The extender pigments used to make flat paints and primers so that they dry to a "matte" gloss is about 10 to 15 microns in diameter.

In order to make paints and primers less glossy, they add extender pigments in the 5 to 15 micron diameter range, and the larger the extender pigment used and the more extender pigment added, the flatter the gloss the paint or primer will dry to. Primers and flat paints both have plenty of large extender pigment in them, so any paint that sticks well to an alkyd primer will stick well to a dead flat alkyd paint, and the same goes for latex primers and flat paints.

It's these extender pigments in the paint or primer that make it dry rough, and that rough surface is what makes the subsequent top coat of paint stick better to the primer than it would the substrate because of the larger surface area that the paint adhesion takes place over.

There is a big difference in extender pigments, too. Often soft materials like calcium carbonate (chaulk) or magnesium silicate (talc) are used as extender pigments because they're inexpensive to grind into a fine powder. However, the mark of a good quality INTERIOR paint will be that it will use a HARD extender pigment like silica dust (which is ground up silica sand). You see, in order to get good "scrubbability" in a paint, which means the ability to scrub the paint hard to remove a stubborn mark without dulling the gloss of the paint, you need BOTH a binder that dries to a strong and hard film and HARD extender pigments. Either one without the other is no good. You need the hard extender pigments to stick out of the paint's surface and not crumble or wear down under hard scrubbing so that they protect the binder film from damage, and you need a strong hard drying binder film to hold the extender pigments firmly in place so that they don't get scrubbed off and out of the paint film. So, if the contents of a can of paint show that the extender pigment used was ground silica, that means that they ALSO would have used a high quality binder that dries to a strong hard film that would support those extender pigments well, and the paint will have good "scrubbability". To use either a hard extender pigment without a hard drying binder (or vice versa) would just be a waste of money on the part of the paint or primer manufacturer.

Extender pigments in primers and paints also serve other useful purposes. Because of the way latex primers and paints form films, there is a tension developed in the latex coating as it dries. Extender pigments in that coating take up space and reduce the tension created. That tension can often be enough to cause a latex paint to pull right off a weak substrate, like chaulking paint.

It's really the rough surface that primers and flatter paints dry to when they have lots of large extender pigments added to them that promote better adhesion of any subsequent top coat. So, it's the fact that primers use a binder resin that chosen because of it's good adhesion that is the reason why primers will usually stick better to bare substrates than paints. And, it's the fact that the primer contains lots of large extender pigments that make it dry to a rough or "dead flat" gloss level that is the reason why paints stick better to a primer than they often do to bare substrates. If you're using a latex paint over an alkyd primer or vice versa, the improvement in adhesion is entirely because of the larger surface area of the primer than a smoother substrate would have. There are no teeth involved.

You can learn more than you need or want to know about latex paints and primers at the web site of the Paint Quality Institute. The Paint Quality Institute was established by and is funded by the Rohm & Haas Company which was recently purchased by DuPont for over 6 billion dollars. The Rohm & Haas company is the largest manufacturer of "acrylic" (pronounced "polymethyl methacrylate" or "Plexiglas") resins used to make acrylic paints, floor "waxes", nail polishes, grout & masonry sealers and acrylic adhesives in North America. So, believe it or not, the paints you buy that say "100% Acrylic" on the can dry to a clear PLEXIGLAS film that only has colour and opacity because of the different pigments embedded in it (very much like raisins in raisin bread).

http://www.paintquality.com/


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## Wisco (Jun 18, 2008)

*Thanks*

Very good I will shut up and listen from now on and value the education.


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## Nestor_Kelebay (Jun 17, 2008)

Wisco said:


> I think the answer is yes. Latex is a superior finish to oil for a topcoat. If oil is used as a primer the thought is wood will absorb the oil and kinda "condition" and seal it. If you have multi layers of build up though I wonder if oil even has a place. JMO


You weren't far off the mark with your first post when you said "wood will absorb the oil", but I wasn't at all sure you knew that there was a difference between how much linseed oil versus alkyd resin wood would absorb.

Nowadays, "oil based" really means "alkyd" even though 20 years ago, it mean "linseed oil based".

True linseed oil based primers are the very best to use over wood because the extremely tiny size of linseed oil molecules allows them to penetrate deep into the wood, and that results in excellent adhesion of the primer to the wood. The problem is that to get that excellent penetration and adhesion, you still have to live with the long drying time and relatively soft film that linseed oil dries to.

Every plant oil and animal fat is a "triglyceride" which means it consists of three "lipids" or "fatty acids" connected to a central glycerine molecule. Each lipid or fatty acid consist of a long carbon chain of 8 to 22 carbon atoms. Normally, plant oils and animal fats are "saturated" which means that every carbon atom in all three fatty acid chains has two hydrogen atoms bonded to it.

However, some plant oils have "unsaturated sites" on their fatty acids. An unsaturated site occurs where two neighboring carbon atoms in a fatty acid chain have only one hydrogen atom bonded to each of them, and there's a double bond between the carbon atoms.

That is, -HCH-HCH-HC=CH-HCH-HCH

where, if you look real close, you can find an "unsaturated site" in that hydrocarbon chain. (hint: it's near the middle)

Where ever you have two such unsaturated sites (on different hydrocarbon chains) in close proximity, then oxygen molecules (O2) from the air can react at those locations and form a PAIR of C-O-C crosslinks between them. That process is called "auto-oxidation" and it's an EXOTHERMIC reaction. If you have an oil with LOTS of unsaturated sites (like linseed oil) then that auto-oxidation of the linseed oil can generate a lot of heat. This is the reason why a pile of rags soaked with linseed oil (or tung oil, or safflower oil or tall oil or even some fish oils) CAN spontaneously burst into flames.

MOST plant oils don't have enough of those unsaturated sites on their fatty acids to ever affect their viscosity. Some oils have more of those unsaturated sites and are said to be "semi-drying". That means that if they are exposed to air for long enough, they will become viscous. Some oils, like linseed oil, Tung oil and all the oils in the list above, have enough of those unsaturated sites on their fatty acids that they will crosslink so densely when exposed to oxygen in the air, that they will actually solidify. Those oils are called the "drying oils".

Todays alkyd resins are best thought of as "clumps" of drying oil fatty acids held together with a synthetic glue.

Thanks to modern chemistry, it is no longer necessary to use expensive oils like linseed or Tung oil to get those all important unsaturated sites. What they do now is use cheap and abundant oils, like soy bean oil, and artificially increase the number of unsaturated sites on it's fatty acids. Then they strip those fatty acids off the central glycerine molecule and combine those souped up fatty acids with glycerine and something called "phthalic anhydride". When you do that, everything reacts with everything else to form a "clump" of those souped up fatty acids.

Take a look at the blue figure on this web page:

http://web.mst.edu/~wlf/CHEM381/chap33.html

(Copy and paste that URL into your browser because this site doesn't seem to recognize it as a link.)

It's supposed to represent an alkyd resin. The two things that look like crabs are phthalic anhydride molecules. The C-C-C connecting them is a glycerine molecule (cuz the reactive hydrogens and hydroxyl groups aren't show) and the thing written as -[CH2]n- which means -HCH- repeated n times represents a fatty acid. To be correct, those -[CH2]n- things would be shown to have lots of unsaturated sites -[HC=CH]- in them.

Aside: and you don't need to know this: Phthalic is short for naphthalic and "anhydride" means "without water". So, you could call KoolAid drink crystals "KoolAid anhydride". So, phthalic anhydride means "what you need to add water to to get naphthalic acid." Anyhow, back to the post.

Those clumps get dissolved in mineral spirits and pigments get added, and when you spread that alkyd paint on a surface, the mineral spirits evaporate first and expose the clumps to oxygen in the air. Once that happens, the O2 molecules in the air find pairs of unsaturated sites in close proximity by the boatload. Pairs of C-O-C cross links form very rapidly both within and between the clumps, causing the alkyd resins (which could flow just like a liquid if they didn't react with one another) to form a more and more viscous fluid until it becomes a solid. Modern alkyds crosslink very aggressively and will dry to a harder film in several hours than the old linseed oil based paints did in several days. And, because it's exactly the same reaction that causes both to form solid films, a pile of rags soaked with alkyd paint and also spontaneously burst into flames.

But, because these "clumps" of fatty acids (called "alkyd resins") are very much larger than linseed oil molecules, they don't penetrate nearly as deeply into wood and don't have as good adhesion to wood as the old linseed oil based primers did.

So, when you said that absorbs the oil of "oil based primers", you were kinda right.

And, if you continue to the next web page on that Chem 381 link, you learn that polyurethane is a plastic made by reacting a "hydroxyl group" (-OH) with an isocyanate group (-N=C=O) with the result that:

A-N=C=O + HO-B result in A-(NH)-(C=O)-O-B

and that ugly thing between the A and B is a "urethane" group.

Now, urethane linkages are very strong, and I really don't think anyone on this planet can explain exactly why they happen to be very strong.

But, if you remember, when we were making alkyd resins, we mixed phthalic anhydride, glycerine and those souped up fatty acids in the pot and they all reacted with each other to form alkyd resins. If you don't already know, glycerine has three hydroxyl groups (-OH).

http://en.wikipedia.org/wiki/Glycerol

So, if we add di-isocyanates or tri-isocyanates to the pot when making alkyd resins, the result will be that the isocyanates will react with the hydroxyl groups on the glycerine parts of the alkyd resin to form urethane linkages WITHIN our alkyd resins. The very first company to do this was the Bayer Company (the Aspirin people) who patented the very first "polyurethane" in 1956, which quickly succeeded real varnish as the clear coat of choice over wood because it formed a harder clearer film.

Thus, the polyurethane "varnish" you buy in a can has tiny clumps of souped up fatty acids in it just like the alkyd resins in an "oil based" paint or primer. The only difference is that the polyurethane resin will have urethane linkages within it that act very much like the roll cage inside a race car, making the resin harder to crush if you could squeeze it, and making it much harder to stretch if you could pull on it. Thus, polyurethane resins are really just alkyd resins with urethane linkages inside them that make them stronger.

The important thing to know, however, is that polyurethanes form films exactly the same way as drying oils and alkyd resins do; by that all important "auto-oxidation" reaction that occurs when O2 from the air spontaneously reacts to form two C-O-C crosslinks between two unsaturated sites in close proximity. And that happens within and between the oil molecules, alkyd resins and polyurethane resins. And, this is also why you can paint a polyurethane top coat over an alkyd paint and get excellent adhesion between the two (because of the formation of those crosslinks between coats as well).

I just thought I'd share.


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## Wisco (Jun 18, 2008)

Nestor, So my neighbor who is staining his neglected cedar sided home said he was going to use oil stain(alkyd), not acrylic latex. Good idea?


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## Nestor_Kelebay (Jun 17, 2008)

I'm presuming you mean he intends to use an oil based solid stain on his cedar rather than what? An exterior latex paint?

I simply don't know enough about solid stains to compare them to paint.

My understanding is that the amount of preparation needed to apply a solid wood stain is less, but I've also heard that solid wood stains have to be applied more frequently than paints.

Sorry, don't know enough about solid wood stains to comment.


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## donkeyot (Nov 20, 2009)

*followup question?*

I am trying to paint some furniture, I have a sample of the paint used on it which smells like pear drops, so I am guessing some alkyd type reaction between acetic acid to form athyl ethanoate ester. So original pain is alkyd based.

So question is a friend has suggested keying original paint then using an acrylic undercoat and oil based topcoat, I am looking for the highest scrubability i can get would a oil based undercoat be better, and what about using a floor paint alkyd primer then a floor paint alkyd topcoat, ie as the primer is developed to adhere best to alkyd paint?


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## moondriver (Apr 30, 2009)

Nestor, 

I'm a general contractor and I use all these products, and I was on the site with the simple question, can I stop using latex primer on bare wood and start using alkyd primer and follow with latex. The problem I'm having is that poplar, which I use most of the time since it's soft and runs through a molding machine nicely, also is extremely absorbant. And when I use latex primer, a good Sherwin Williams, it still raises the grain, and I have to sand before top coating with semi gloss latex, again Sherwin Williams Pro Classic. So I wanted to see if the latex topcoat would bind to the alkyd primer. 

You not only answered my question, but I feel like I got a good education as well. You must teach chemistry, or else you've been in industrial research. However you've accumulated your knowlege it's substantial, and I know everyone, especially myself, appreciated your taking the time to write so extensively about how it all works. I read all your posts with fascination; learned a lot, and wanted to thank you for taking the time to put it all into a post. I know how long it takes to write that way on a subject, but it was a brilliant essay. And I feel like I got a real good education.

Thanks for posting it!

By the way, solid stains don't hold up worth a damn. You have to really redo them every year. I have no idea what goes on at the molecular level, but it's probably something along the lines of what you were talking about with Linseed Oil. I would guess the molecules are so tiny they seep down into the grain, especially with a porous wood like cedar, and what's left on the surface is so thin it wears away. But I know this strictly empirically.


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## Matthewt1970 (Sep 16, 2008)

I personally will only use Oil on bare wood. It penetrated deeper than latex, is more duarble than latex and will stop wood bleed where latex won't. It will also lay down smoother and not lift the grain.


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## moondriver (Apr 30, 2009)

Thanks Mattthew1970,
I have some good Sherwin Williams alkyd primer and will use it. The other problem with the latex primer is that it took two coats to cover the wood, and then I had to sand before putting on the gloss or else brushstrokes showed through.


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## Matthewt1970 (Sep 16, 2008)

Oil will sometimes do that but that is just due to it soaking in, which is what you want.


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## spamuel89 (Sep 27, 2021)

Nestor_Kelebay said:


> You weren't far off the mark with your first post when you said "wood will absorb the oil", but I wasn't at all sure you knew that there was a difference between how much linseed oil versus alkyd resin wood would absorb.
> 
> Nowadays, "oil based" really means "alkyd" even though 20 years ago, it mean "linseed oil based".
> 
> ...


Dude I have been loving these posts. An awesome bunch of info for me to nerd out on and fill so many gaps in my understanding. I really can't thank you enough for having taken the time to post all this. It's never been so thoroughly explored in a place i could find. I work doing painting and construction and literally no contractor ever bothers to know even a rudimentary level about this stuff. So I end up sounding pompous when I ask questions trying to get more in depth


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