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my sweet wife insists on applying oil-based paint to the latex already there, saying she has "sanded the hell out of it," so it must be okay. I always heard we just don't do this, but I cannot tell her why. Anybody know?:confused:
I am assuming you are talking about interior woodwork. In this case, it is perfectly fine to go over latex with oil products. The usual concern is with the reverse - latex over oil, because of concerns with getting good adhesion. Earlier generation latex paints did not adhere well to oil. However, there are now self-priming acrylic paints which will go directly to oil.
I often went over poor past latex jobs with a coat of heavy build oil enamel undercoater followed by a coat of oil enamel. The undercoater helped hide the "ropiness" of the latex and seal it. With a coat of lower sheen enamel, the woodwork would be greatly improved in appearance.
On exterior surfaces, mixing oil and latex products is not generally a good idea because they expand and contract at different rates during the heat variations encountered outside. Oil is hard, brittle and relatively stable. Latex/acrylic expands greatly. The "tug of war" created can actually cause massive paint failure, with sheets of paint being pulled right off the wall!
Besides, oil paints yellow in the sunshine, changing the color
I agree with everything said above, but there is one more thing to be said. Due to the EPA, CA and AZ, oil paints are not what the use to be. The VOC'S are not as good as before. I am sorry for all the intials. The out come is that while latex paints are getting better, oil paints are getting worse. Go with a good latex trim paint and it will last longer than oil.
And even as the paint manufacturers have taken the VOC's out, painters have added the Penetrol and mineral spirits right back in! It is kind of like the gun catalogs that sell non-operable machine guns, and right next to them, the kits to make them fully automatic. Kind of an end run around the laws.
I still prefer a good oil over latex for fine woodwork. It still dries to a much harder, less gummy, smoother finish, especially if brushed. If knicked, it feathers out with sandpaper and can be touched up imperceptibly. Latex/acrylic just clogs your sandpaper.
I will agree that whites still yellow.
All this having been said, the writing is on the wall. :(
Ordjen, You are correct in the fact that most latex paints will gum up sand papper, but have you tried Kelly-Moore Duropoxy or SW's Pro Classic water based? I have sanded both with in one hour with great results. If oil yellows, how can you touch it up after a year or more???. If you have an oops, latex can be fixed same day, oil you have to wait a day before recoating.
The danger is that in having been away from contracting now for several years, I no longer try out different new materials as I once did. I am sure that water based paints continue to get better and will one day obtain all the positive attributes of ol paints. One of my favorit water based paints was the Muralo line. Muralo is best known for owning the trade name "Spackle", but it also manufactures very good paints which dried relatively hard and were sandable. Unfortunately, it is one of those regional manufacturers which does not market across the whole country.
Touch-ups at a later date was one of the problems with oil paints. After only several months, kitchen cabinets would have yellowed to the point where a significant touch-up would be noticeably yellower.