Primer Peeling


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Old 11-08-10, 08:57 AM
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Primer Peeling

I recently primed some new drywall in the bathroom and while I was tiling the shower, the primer began to peel in areas when cleaning the tile with a wet sponge. Is this normal or is the primer I'm using no good? It's not peeling too bad, more like rubbing off.
 
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Old 11-08-10, 01:12 PM
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Did you remove the sanding dust prior to applying the primer?

Which primer did you use?
 
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Old 11-08-10, 01:40 PM
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Sorry, I'm not understanding - is the primer under the tile or are you scrubbing the primer when cleaning the tile since it's adjacent? How long had the primer been on the wall before this started happening?
 
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Old 11-08-10, 02:21 PM
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Mossman:

No, you're probably OK. Just turn on the bathroom fan and allow the air in the bathroom to dry out, and see if the primer returns to normal.

You see, general purpose primers are made from a plastic called polyvinyl acetate, or PVA for short. You probably know PVA better as white wood glue. And, if you're familiar with white wood glue, you'll know that furniture repairmen will take wooden joints apart that were glued together with white wood glue by steaming the joint to soften the glue.

Well, the same characteristics that you find in PVA white wood glue you also find in PVA primers and paints. They don't tolerate high humidity or moisture well. When a PVA primer gets exposed to high humidity or moisture, it softens up and loses it's adhesion. The result is that the primer will crack and peel. Once the primer is covered with a paint, then the problem is eliminated because the paint somewhat protects the primer from exposure to high humidity and moisture.

Just let the bathroom dry out, and the primer should return to normal. If it doesn't, post again.

What you should do is paint over your primer with a paint made exclusively for bathrooms, like Zinsser PermaWhite Bathroom Paint or Sherwin Williams Bath Paint. Both of these paints will use a different kind of plastic as the binder, namely polymethyl methacrylate, or PMMA, which you probably know better as Plexiglas.

Paints that use a Plexiglass binder are referred to in the industry as "100% Acrylic" paints, whereas primers and paints that use PVA as the binder are referred to in the industry as "PVA" or "vinyl acrylic" primers and paints. PMMA binders are very much more resistant to moisture and high humidity than PVA binders.

So, if you top coat with a bathroom with a paint made specifically for bathrooms, not only will you be getting a PMMA binder, but the binder you'll get will be chosen because of it's high resistance to moisture and humidity.

If the primer doesn't return to normal when the room dries out, then post again. Otherwise, carry on.
 
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Old 11-08-10, 03:09 PM
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Nestor, thank you very much for the info. The moisture due to the tile glue and cleaning during application is definitely the cause then. I'm sure it will be fine. Thanks again.
 
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Old 11-08-10, 04:38 PM
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Mossman:

I once found that I could scrape PVA paint off a wall with my fingernail after cleaning the wall with a dilute solution of Mr. Clean. The following day, after the paint had dried up, it returned to normal, and I wasn't able to do that. So, that "rubbing off" you noted is almost certainly due to the softness of the PVA film while it's wet or in a highly humid environment.

Once the primer is covered with a good quality PMMA paint, preferably a paint made specifically for bathrooms, you won't have any problems with peeling or the paint softening up. You should reprime those spots where the primer did actually peel off tho.

Over and out.
 
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Old 11-08-10, 05:05 PM
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By the way, the product is Valspar Interior Latex Primer. Do you recommend repriming with an acrylic primer? FYI, I will be using Behr semi-gloss for the bathroom ceiling and satin for the walls.

Shop Valspar 5-Gallon Interior Drywall Primer at Lowes.com
 
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Old 11-08-10, 07:41 PM
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I tried to get onto Valspar's web site to see if I could download an MSDS or Technical Data Sheet for their drywall primer just to see what was in it, but my computer kept freezing up every time I tried to access their web site.

Personally, I think it's far more important for you to use a paint made specifically for bathrooms in your bathroom than it is to reprime with a different primer.

I own a 21 unit apartment block in Winnipeg, Manitoba. You can see pictures of my work at:
Apartment rentals in Winnipeg, Manitoba
In all 21 of my bathrooms I used Zinsser's Bullseye 123 as the primer, which is one of the few examples of a styrenated acrylic binder you can find (which is a relatively uncommon third kind of binder used in paints and primers, other than PVA and PMMA). And, I used Zinsser's PermaWhite Bathroom Paint, which is a PMMA binder with very high water resistance. And in over 20 bathrooms in darn near 20 years now, I haven't had any paint problems whatsoever. (And I think I can guarantee that it's not because all of my tenants have religiously wiped all the moisture up on the walls and ceilings after each bath or shower. Most of my tenants wouldn't bother turning off the bathroom light after showering.) Still, the paint has held up well, with no mildew growth, cracking or peeling.

You see, formulating a paint meant for bathrooms isn't just a matter of picking the acrylic binder that has the highest water resistance. That acrylic binder also has to allow mildewcides in the paint to migrate through the binder film toward the wet surface of the paint. These mildewcides are extremely soluble in water, and it's that high affinity for water that causes them to move toward the moisture when the paint gets wet. Once at the surface they dissolve in the water, and either are removed if that water is wiped up, or remain behind on the paint if the water is left to evaporate. It's the mildewcide that actually leaches out of the paint onto it's surface that kills mildew spores that land on the paint before they grow, thereby keeping the paint free of mildew.

Take a look at Figure 6 in this article:
Chemistry is Key to Microbiology for Coatings - Feature Articles - Paint and Coatings Industry

It shows the rate of mildewcide leaching for several different kinds of mildewcide in flat, semigloss and gloss paint. When it comes to mildewcide leaching, the gloss level of the paint is a very important parameter because it determines the surface area of the paint that gets wet, and hence both the driving force causing the mildewcide to move toward that wet surface, and the surface area through which the mildewcide can leach out of the paint film. Thus, a flat paint can leach out more mildewcide faster than a glossy paint. You can see in that chart that for all three mildewcides used in flat paint, all the mildewcides would leach out of the paint rapidly, leaving the paint unprotected against mildew within a short while. For the one example of gloss paint, the mildewcide leached out of the paint so slowly as to be ineffective in keeping the paint film free of mildew. The appropriate choice of mildewcide has to take into account the gloss level of the paint so that the mildewcide will leach out of the paint at the slowest rate which is still 100% effective in keeping the paint film free of mildew. In that chart, the semi-gloss paints would provide better performance than either flat or high gloss paint using those three mildewcides.
(That article mentions "fence tests". That's because this article is actually talking about mildewcides used in general purpose exterior latex paints. Exactly the same sort of testing is done with different gloss interior paints to determine which combination of binder, mildewcide and gloss provide the longest term protection against mildew in bathroom paints.)

Now, I'm sure that you'll be happy with your Behr paint in the living rooms, hallways and bedrooms of your house. However, that Behr paint was formulated to be a general purpose wall paint which wouldn't be expected to get wet often. It wasn't designed to perform well under wet and humid conditions like you find in bathrooms. It's binder wasn't chosen because of it's excellent water resistance. It won't have hardly any mildewcides in it at all, let alone a mildewcide that was chosen to provide the slowest effective leaching rate possible. Your Behr paint simply was never designed to outperform regular paints under moist and humid conditions like you find in bathrooms, and so don't expect it to.

I wouldn't be taking extra time out of my day to convince you to use anyone's paint made specifically for bathrooms unless I thought it was important to do so. But, at the end of the day, it's your bathroom, it's your money and it's ultimately your choice. I'm giving you the best advice I can, but ultimately you have to decide whether or not to take it. Generously donate any Behr paint you've already purchased to the next squeegee kid that cleans your windshield, or wait until it gets dark and no one's around and go throw it in the river, but at the end of the day, you'll have a better bathroom if you use a paint in it that was designed specifically to perform well in bathrooms.

Use a bathroom paint. Put on at least two coats of bathroom paint (and three is better) to provide a large reserve of mildewcide. Don't clean the bathroom walls any more often than is necessary. Never paint over the bathroom paint with anything other than a bathroom paint. If you follow these commandments, the paint on your bathroom walls will outlast grandma.

And, if all of this has fallen on deaf ears so far, let the numbers convince you. I'd say it's reasonable to expect that the average home owner repaints his bathroom every 20 years. Well, my bathrooms have to last way longer than that. If they didn't, then by now in a building with 21 bathrooms, I'd be repainting a bathroom every year. And, that wouldn't allow time for the 1001 other things I do in my building. So, my bathrooms have to significantly outlast other DIY'ers bathrooms because I have 21 times as many bathrooms to maintain as other DIY'er do.

The defence rests.
 

Last edited by Nestor; 11-08-10 at 09:16 PM.
  #9  
Old 11-09-10, 05:16 PM
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Should I go ahead then and finish painting before I grout the tile? This way I wouldn't have to worry about more primer rubbing off while I'm grouting around the perimeter with a wet sponge.
 
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Old 11-09-10, 08:09 PM
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Normally you'd finish the tiling, then paint up to the edge of the tiling, then grout.

That way, if you chose not to use bullnose tiles, you could just run a fillet of grout between the tiling and the paint. I do that on all my tiling, and it looks fine.
 
 

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