Why must exterior house paints be oil-based?

Exterior house paints must be oil based because of the weather. If a water based paint is used, the paint will be damaged when it rains. Using a water based paint would be like coloring the outside of your house with washable markers.

Exterior Paints DON'T Have to be Oil Based

The vast majority of exterior paints are water-soluble. Though water is the carrier, once an acrylic latex paint cures, it is no longer soluble in water, so the notion that it's like a washable marker is ridiculous. In testing, water based paints often outlast oil based paints because they form a non-reacting, flexible covering, while oil based paints are rigid (so they crack with expansion and contraction) and continuously oxidize, leaving a chalky surface.

Oil based paints can leave a nicer finish, particularly when glossy, because they flow better and dry more slowly than latexes. If you already have several layers of oil-based paint on, it's best to stick with it. Other than that, water based paints are better for most applications, interior or exterior.

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I am in agreement...No Oil On Exteriors...you can't buy oil-based paint in volume anymore thanks to "o.s.h.a.".

Exterior latexes have elasticity and conform to weather changes. You can buy "Floetrol" if you're having a leveling problem. This reduces the viscocity of the paint thereby allowing it to lay down and self-level. If you're having the same problem with oil, use "penetrol" or "naptha".

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In some applications, exterior paints are better!

Don't use latex paint. Latex paints are a marvelous invention for many reasons but, in some applications and under some conditions, gloss and semi-gloss latex paints can develop a "tackiness" that seems to last forever. I painted a garage door with an exterior latex paint over 5 years ago; in the warm weather, I still hear the sound of the tacky paint breaking contact as the door goes up!

The problem is known as "blocking", and can be caused by many factors such as applying an overly thick layer of paint, not allowing adequate drying between coats, reactions between the primer and the finish coat or the temperature at which the paint was applied -- too hot or too cold.

The best way to avoid blocking is to use an oil-based exterior alkyd paint instead. Alkyd paints, which are the top quality oil paints, dry hard, smooth, and do not produce tackiness. There are mixed views among painters on preparation for a latex-to-oil changeover. Oil paint can be applied over latex without priming as long as the environmental conditions are not too demanding. A light sanding OR the use of a "deglosser" such as Wilbond is essential for a firm paint bond. In the case of exterior doors, I vote for (at the least) sanding followed by priming the doors prior to applying the oil. The primer must be an exterior grade and can be either oil- or water-based. Again, there are some strong opinions on using either type of primer. Be sure it is an exterior grade. Under NO circumstances use Kilz, BIN, or any of the other interior primers.

Depending on location of painted surface, wood can become hot enough under exposure to the sun to blister paint and distort certain plastic moldings that some companies use to mount the window glass! This can cause the paint to prematurely fade or appear weathered. In fact, this overheated situation can even cause wood to expand and contract severely enough to cause cracks! And, guess what makes the situation even worse? Dark pain colors!

A final comment concerning blocking: if you are not ready to repaint and you would like to eliminate any tackiness, one method that I have used successfully is to apply auto or paste wax to the sticky area. The wax coating removed the tackiness, though it may have to be repeated occasionally when any tackiness reoccurs.