# Does primer need to be sanded?



## Yirg (Apr 12, 2015)

As part of my projection wall project I applied a layer of joint compound and sanded it, then applied a layer of primer (using a short nap roller). It doesn't feel as smooth to the touch as before. Does this mean I should sand it before painting?

Thanks!


----------



## Jmayspaint (May 4, 2013)

Yes, it's standard to practice to sand the primer coat for smoothness.


----------



## Yirg (Apr 12, 2015)

Thanks, I did that now and it was far less dusty than sanding joint compound or plaster. There's still some flashing, mostly because I did a poor job with the joint compound. I wonder if another layer or primer + sanding would reduce that?


----------



## expertPainter (Sep 15, 2014)

Yirg said:


> Thanks, I did that now and it was far less dusty than sanding joint compound or plaster. There's still some flashing, mostly because I did a poor job with the joint compound. I wonder if another layer or primer + sanding would reduce that?


Use a high quality flat


----------



## Jmayspaint (May 4, 2013)

What do you mean by flashing? You can see the outlines of patches or the up and down strokes of the roller? Which primer did you use, 123?


----------



## jeffnc (Apr 1, 2011)

Wait, this is a projection wall? Like for playing movies? That would require some special attention. More details needed....


----------



## Yirg (Apr 12, 2015)

Jmayspaint said:


> What do you mean by flashing? You can see the outlines of patches or the up and down strokes of the roller? Which primer did you use, 123?


I mean that the layer of joint compound is not even enough, and in some areas the plaster below is showing through. I guess I should have applied another layer of compound before adding the primer. Not sure what would be the best course of action now. I'd rather not go through additional steps if I can get a reasonable result by just applying another layer of primer and sanding it, then painting.

The primer I'm using is from a local Israeli brand (Nirlat Primer S). It's water based for use with acrylic paint. It's transparent with satin finish.



jeffnc said:


> Wait, this is a projection wall? Like for playing movies? That would require some special attention. More details needed....


Yes, that's the intention. Please let me know what additional details are needed.

Thanks!


----------



## jeffnc (Apr 1, 2011)

I would start with this article. You can search the web for others.

http://www.projectorcentral.com/paint_perfect_screen_$100.htm?page=DIY-Screens

On Page 4 (Paint the Screen), it says "1. After the paint on the surrounding wall outside the screen area has dried, re-apply masking tape. This time, lay the tape on the _outside_ edge of the image, on the newly painted surface."

I would caution you to wait several days before putting tape on a newly painted wall, not just until the paint is "dry", or else the tape will take the paint off.

It then says "3. Apply a coat of primer... Allow the primer to dry."

It is at this point I think we all agree to lightly sand the primer (wait until it is very dry, preferrably about 24 hours, depending on directions on the can). Presumably you will be putting on the primer with the same roller cover they recommend in the article for the finish paint, but still I'd sand.


----------



## Nestor_Kelebay (Jun 17, 2008)

With utmost respect for all the knowledgeable people in here, I don't see the point in sanding a primer. Can someone please explain the benefit to be had in doing that?

Primers have huge rocks in them that are almost large enough to see with the naked eye. The purpose of those rocks is to make the primer dry to a rough surface. That increases it's surface area so that the top coat of paint will stick better. So, if the primer is already rough enough for paint to stick well to it, why sand it to make it rougher?

That is:

If the primer surface looks like this:

vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv

and you sand it to make it look like this:

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

you've made the surface rougher, but you're not increasing the surface area the top coat of paint has to stick to, and that's what matters. If it's the same surface area in both cases, the paint will stick equally well in both cases.

Presumably the primer dries rough enough for excellent paint adhesion to it. Otherwise it wouldn't be a very good primer.


----------



## jeffnc (Apr 1, 2011)

First of all, the second example you showed does have a higher surface area. It's about double.

The purpose of sanding primer is to make it smoother in the macro sense. i.e. if applied with a roller especially. We're taking off the tops of bumps larger than the ones shown in your diagram.


----------



## Jmayspaint (May 4, 2013)

The idea is not to sand for the purpose of adhesion/roughness but for uniformity and smoothness. It's not as important over full skim coats, but partial skim or level 4 drywall it really helps to even out the surface. Any exposed drywall paper will have a different texture than the mudded areas and if the paper was damaged during sanding, it gets quite rough. Sanding helps equalize these textural differences. 

Sanding the primer on completely smooth mud will technically roughen the film, but it also helps minimize roller stipple and any unevenness left from the application. So the net effect is smoother overall. It can also help keep flashing from telegraphing through the top coat with primers that have a sheen like 123. 

Assuming a perfect application, say a sprayed primer over level 5 drywall, sanding would serve no purpose and may actually be counter productive. But most of the time these perfect conditions don't exist, and sanding the primer coat is beneficial to the uniformity of the top coat. 

That's how it seems to me anyway.


----------



## jeffnc (Apr 1, 2011)

Jmayspaint said:


> Assuming a perfect application, say a sprayed primer over level 5 drywall, sanding would serve no purpose and may actually be counter productive.


That's true, although we shouldn't be sanding with 60 grit before the final coat. I think if we're sanding with 120-150 grit and acrylic paint with reasonable solids, then the "gouge size" is going to be completely filled in with paint, becoming unnoticeable. So spraying over Level 5 would give the absolute smoothest finish possible, but I don't think a high grit sanding would hurt that unless the paint were thinned (or started with a thin paint) and then the paint were sprayed very thin.

Has anyone actually seen 150 grit sanding marks show through wall paint?


----------



## jeffnc (Apr 1, 2011)

Jmayspaint said:


> It's not as important over full skim coats, but partial skim or level 4 drywall it really helps to even out the surface. Any exposed drywall paper will have a different texture...


The texture of unskimmed drywall paper is clearly visible with the naked eye (in the right light) underneath a sprayed finish, or underneath any thin-ish paint film without a lot of roller texture.


----------



## jeffnc (Apr 1, 2011)

jeffnc said:


> http://www.projectorcentral.com/paint_perfect_screen_$100.htm?page=DIY-Screens


By the way, using the ProClassic in waterborne alkyd will probably give you an even better finish than the acrylic version they tried.


----------



## expertPainter (Sep 15, 2014)

jeffnc said:


> That's true, although we shouldn't be sanding with 60 grit before the final coat. I think if we're sanding with 120-150 grit and acrylic paint with reasonable solids, then the "gouge size" is going to be completely filled in with paint, becoming unnoticeable. So spraying over Level 5 would give the absolute smoothest finish possible, but I don't think a high grit sanding would hurt that unless the paint were thinned (or started with a thin paint) and then the paint were sprayed very thin.
> 
> Has anyone actually seen 150 grit sanding marks show through wall paint?


Yes, I've seen 150 grit show through. 

Sanding serves 2 purposes.
1. Creating a smooth to the touch/feel surface. 
2. Creating peaks and valleys for the finish coat(s) to grab and hold on to while they're applied.


----------



## jeffnc (Apr 1, 2011)

It seems to me that if you're seeing 150 grit through decent acrylic paint, then someone must have had a very heavy hand. But I could be wrong.


----------



## expertPainter (Sep 15, 2014)

jeffnc said:


> It seems to me that if you're seeing 150 grit through decent acrylic paint, then someone must have had a very heavy hand. But I could be wrong.


13 years in the biz, you see pretty much everything.

You name the substrate, I've painted it, or know how to without any issues


----------



## Yirg (Apr 12, 2015)

jeffnc said:


> I would start with this article. You can search the web for others.
> 
> http://www.projectorcentral.com/paint_perfect_screen_$100.htm?page=DIY-Screens


I'm familiar with this article and it's very informative, but its recommendation to use satin finish paint probably doesn't apply to my case, where there are imperfections that I'd like to hide. Starting over and simply isn't an option at this stage. Applying and sanding joint compound are steps I really want to avoid. I hope that matte/flat finish will be sufficient. Frankly, the projected image quality was very impressive even before I embarked on this project, and the wall far from smooth. It should be much better now that the wall is relatively smooth and just needs to be painted.



> It then says "3. Apply a coat of primer... Allow the primer to dry."
> 
> It is at this point I think we all agree to lightly sand the primer (wait until it is very dry, preferrably about 24 hours, depending on directions on the can). Presumably you will be putting on the primer with the same roller cover they recommend in the article for the finish paint, but still I'd sand.


Unfortunately I already sanded the primer (which was only seemingly dry) before reading your post. It felt a bit sticky right after sanding, but not any more. It feels dry and smooth now. Beginners luck I guess...


----------



## Oden (May 23, 2015)

I use 100 grit on a sanding head. Just drag it over the primer. Not hard just drag it really. Let the paper do the work.


----------



## ToolSeeker (Sep 19, 2012)

First primer will not cover like paint, it's not supposed to. So yes you will be able to see spots thru it and it will look streaked. It's job is not to cover but to give an even surface. Joint compound will absorb differently from the drywall, primer will even this out.

Using a roller cover with a nap will leave stipple (roughness). You can sand the primer perfectly smooth, but if you are going to roll on your finish the stipple will return. Some say you can use a foam roller cover to eliminate the stipple, this has NEVER worked for me. In my opinion the only way to eliminate stipple would be to spray.


----------



## Yirg (Apr 12, 2015)

ToolSeeker said:


> You can sand the primer perfectly smooth, but if you are going to roll on your finish the stipple will return. Some say you can use a foam roller cover to eliminate the stipple, this has NEVER worked for me. In my opinion the only way to eliminate stipple would be to spray.


Probably a stupid thing to ask, but can you sand that stipple away (referring to the final paint layer)?


----------



## ront02769 (Nov 28, 2008)

Sure you can't sand it away....but of course the aint will end looking like crap. Ron


----------



## ToolSeeker (Sep 19, 2012)

No you can't sand the final finish it would remove any sheen from the paint and it would really look bad. Even when we spray we do what is called backrolling. What that means is after we spray we take a roller and run over the wet paint. The purpose of this is to put stipple on the wall.

The reason we do this is if that wall ever needs touched up it could be done with a roller and the stipple would match. If we didn't do this, and the wall was to ever need touched up it would have to be re-sprayed to be smooth again. If you touched up a sprayed wall with a roller it would stick out horribly.


----------



## Yirg (Apr 12, 2015)

ToolSeeker, you wrote "the only way to eliminate stipple would be to spray" and then explained that "after we spray we take a roller and run over the wet paint. The purpose of this is to put stipple on the wall"... if you end up intentionally adding stipple, wouldn't it be easier to just paint with the roller to begin with?


----------



## jeffnc (Apr 1, 2011)

When you paint, you need to accomplish 3 things:
1) get paint from the container to the surface
2) "work" the paint into the surface so it adheres and covers well
3) give the paint the final surface appearance you want

You don't have to do all three of those with the same tool. For example, house painters often paint wood siding by applying with a sprayer (much faster than moving paint from the can with a brush), and then brushing (to work the paint into the wood grain which a sprayer can't do.) Use the right tool for each job.

When painting a wall, it's not that important to work the paint "into" the existing paint on the wall. Just the little force of the paint droplets flying through the air is enough to make it stick and cover well. If you want a smooth finished surface, then spraying can accomplish all 3 things at once.

It's really fast to spray paint on a wall. It's also pretty fast to roll when you don't have to keep going back to a tray. If you want a stippled finish and you have enough wall surface to do so that you don't mind spending the overhead in time to clean your spray equipment, then doing it that way is actually pretty efficient. Not that it's _cost _efficient for a homeowner to buy a good sprayer for painting a couple rooms a year at most.


----------



## Yirg (Apr 12, 2015)

I only need to paint a single wall, so although spraying sounds perfect and would give smoothness that's ideal for projection, it's probably not cost effective in my case to purchase the related equipment. I guess a short nap roller is the next best option if I want to get as little stipple as possible...


----------



## jeffnc (Apr 1, 2011)

Basically I agree, although you might want to look into something like this.
http://www.homedepot.com/p/Preval-9-oz-Complete-Spray-Gun-267/202533738


----------



## Yirg (Apr 12, 2015)

I found a relatively inexpensive (but brandless) sprayer which may be sufficient for this project. I created a separate thread to get opinions in the painting forum, here.


----------

