Replies

1. 
   davidmeiland | Nov 06, 2004 02:27am | #1

   I would call tech support at USG and talk to them... and/or... call tech support at Benjamin Moore. There is also the very knowledgeable Myron Ferguson over on the JLC drywall/finishes forum. Throw a question on that board and see what they say.

2. 
   Piffin | Nov 07, 2004 06:10am | #2

   and then come back here with the answers to help us all learn

    

    

   Welcome to the
   Taunton University of Knowledge FHB Campus at Breaktime.
    where ...
   Excellence is its own reward!

3. 
   kostello | Nov 07, 2004 11:54am | #3

   just an educated guess but i reckon that the issue is not with the pva sealing the plaster but giving it a 'shiney' slick finish that paint won't slick to very well.

   if that's the case you might be able to lightly sand it all with 80g paper to give the paint a key.

   ????????????

   just a thought

4. 
   user-331077 | Nov 07, 2004 05:44pm | #4

   its suggested to let the plaster outgass for 30 days b4 priming .  you'll most probably be ok but a few areas may need touch ups .  go with it don't look back. damn the critics and throw out the directions .

1. 
   almaltd | Nov 07, 2004 07:50pm | #5

   I am not certain about this, but, I think it is the salts in the plaster that causes the problem. Even over 80 year old plaster I use a primer/sealer that my paint supplier recommends, in my case it is General Paint Super Seal (Canadian product)

   ALMA

5. 
   BarryE | Nov 08, 2004 12:03am | #6

   the PVA wouldn't be my first choice for plaster, and if i were to use it I'd want to make sure the plaster was totally cured, but you might get by......Try the scratch test to see how well it's adhering to the plaster.

   PVA is polyvinyl acetate (white glue) based and the ones I've seen say they can be used on plaster as long as it is cured..cured being the key word.

   Here's what USG has to say:

   Polyvinyl acetate (PVA) based primers should not be used over wet plaster of any kind, over limegauging, or over lime-containing plasters. The PVA film is subject to rewetting and will almost certainly result in bond loss and subsequent paint delamination. There are three main reasons for this.

   First, a PVA primer is by its very nature hydrophilic, that is, tending to combine with or dissolve in water. Thus it is highly susceptible to swelling when wetted and to shrinking when redried. Second, if not allowed to totally evaporate prior to priming, moisture in the plaster assembly plus the added absorbed water from the primer application must dry after the primer film has formed.

   Although PVA films are permeable, when applied to wet plaster the drying rate of the plaster is impaired. This creates a condition which allows dissolution of the lime, as hydroxide, and of the gypsum; these substances migrate and deposit themselves as minute crystals at or near the surface of the plaster finish. A powdery film forms which interferes with effective primer adhesion.

   Third, since PVA films are rewettable and permeable, application of a water-based paint can contribute to bond failure, since moisture provided by its application can both distort the primer films and promote further formation of lime/gypsum crystal deposits.

   Here's what the back of one PVA primer says:

   Surfaces: Interior drywall, plaster, concrete, insulation,

   previously painted or primed metal. Not recommended

   on lime containing plaster, floors, or decks.

    

   And another:

   PVA is a fast drying, vinyl acrylic wall primer for interior walls and ceilings. PVA is specially recommended for unpainted drywall surfaces as it will not raise the surface fibers. PVA is also used on new or previously painted plaster, masonry or tight wallpaper. Product may be topcoated with either latex or oil based finishes and features quick dry and water clean-up. PVA can be applied by brush, roller or spray.

   ---

   Barry E-Remodeler

    

1. 
   OregonBob | Nov 08, 2004 09:11pm | #7

   So, I don't get it.  Is the veneer going to come off the blue board, or is the paint going to come off of the primer? 

   Why would the paint come off of the primer when it's applied to a plaster wall, but not when it's applied to a drywall surface?  Around here, everyone uses PVA as a primer over drywall because it is approved as a vapor barrier method.

   Oh well, too late now.  I'll just have to wait and see what falls off.  No problems so far, though.

1. 
   jackplane | Nov 08, 2004 11:26pm | #8

   I think the paper on the drywall acts as a barrier to prevent gypsum migration, which could interfere with the bond of the PVA in plaster.

2. 
   BarryE | Nov 09, 2004 12:46am | #9

   i think your main concern would be if the plaster was still wet.

   try scratching it, if it's bonded to the plaster and there are no "burn" spots bleeding thru the primer..... paint it and forget it.

   Barry E-Remodeler

    

6. 
   Frankie | Nov 09, 2004 04:06am | #10

   I read what Barry E wrote. Fine explaination and chemistry. Here's my experience.

   The bond failure will occur between the PVA primer and the plaster. The PVA primer will not fail until it is topcoated. Then it will/ may bubble up from the plaster. Basically it has little bonding strength (keying) to the plaster and when it is wetted by the subsequent coat of latex paint it swells and therefore bubbles off. You can then peel it off in sheets. Are we having fun yet?! But then again, it may not. Ya don't know until it happens. Your plaster may be dry enough and not porous enough to react.

   With alkyd (oil) primers, the primer actually soaks into the plaster and therfore has a much stronger bond and seals the plaster chemistry from exposure to other elements.

   There is nothing you can do at this point. Don't do any more PVA priming on plaster but go ahead and continue to paint. Other than another layer of GWB, there is nothing you can apply to the PVA primer to prevent delamination or to create a stronger bond between the PVA primer and the plaster.

   F

1. 
   HeavyDuty | Nov 09, 2004 05:46am | #11

   If it's not broken don't fix it.

   So leave the PVA primer as a finish coat. :)

   I am wondering if applying an oil based primer over the PVA would solve the delamination problem. Oil would not rewet the PVA and then he can just top coat it with either oil or latex.