Originally Posted by Paintguy
Actually the liberal use of extenders/water does hurt the film since they are lowering the volume solids. What is means is that with each coat, you are putting less on the substrate. On interior coatings, even minor amounts of these products can dramatically decrease dry surface performance (scrubs, burnish resistance, color rub off). The old adage holds true, you do not get something (open time) for nothing. If it were that easy, the manufacturer would simply put it in the paint.
All that said, it's a moot point for a roof - like Ric said, it's going to fail no matter what you do.
Whenever I make comments such as "There really is nothing in latex extenders that'll hurt your paint if you use it "liberally"..."
, I do so with the assumption that everyone knows what is going on in my mind at the time I say it...I'm also one to borrow and paraphrase old adages (as it relates to paint), my favorite is "For every advantage there is an equal and opposite disadvantage". So, you're right - kinda. The result of adding a latex extenders to latex paint might result
in some of the issues you've listed - just as they might
experience issues I alluded to in my post. I make those comments based on a compositional perspective - the make-up of latex extenders are components already found in the coatings they are attempting to extend - they are (as they relate to the coatings) inert and innocuous and will have no detrimental effect to the resin of the coating...
...and, from a manufacturer's perspective, products are formulated to perform in a wide range of applications and exposures. It'd be literally (and economically) impossible for a manufacturer to create the perfect recipe for a product to be used in each type of application, for every type of exposure, to endure each atmospheric influence...Therefore, various additives and extenders are sometimes necessary AND many times endorsed (albeit unenthusiastically) by the paint manufacturer.
Finally, I never said "it's going to fail no matter what you do" - I said it's not an ideal situation...Roofing shingle can be painted successfully with surprisingly good, long lasting results. The caveat to the person painting the roof is when using standard house paint variety products, is this is not a fix to roof in need of repair - it is simply aesthetics (although there are some "paint" type products that can do temporary structural repair to a roof).
I appreciate your comments, and we're really not that far apart in our philosophies.