prepping old wood for painting newb questions
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.
Last edited by Nestor_Kelebay; 08-07-2008 at 12:28 PM.