Tim Carter talks about house paint and the differences between acrylic polymer and vinyl polymer. The folks at This Old House point out these additional things to look out for when picking an exterior paint:
Proper pigments. Quality pigments allow a good paint to cover fully with just one coat. Paints with lower-cost pigments often must be applied in several coats. That means more work, which makes buying low-quality paint a poor financial decision. The best pigment is titanium dioxide. Look for it when ingredients are listed on the can.
High percentage of solids. The solids are what's left on the wall after the paint has dried. Anything over 45 percent is considered good; the higher the level of solids, the better, because you'll wind up with a denser, more durable coating.
All-acrylic binder. The binder is what holds the pigments, mildewcides and other solids that form the actual paint film. Look for latex paint with an all-acrylic binder, which is inherently more weather resistant than vinyl or vinyl-acrylic.