You may want to choose your paint based on what was used before. As with interior paints, latex works best over latex and alkyd works best over alkyd. If you can't tell or are unsure about what type of paint is on the house, use an alkyd-base paint.
Latex paints are easier to apply, dry quickly, and can help minimize moisture problems because they "breathe." Cleaning up is a matter of soap and water. These paints do not adhere as well to oilbase or alkyd-base paints or to poorly prepared surfaces, however. Alkyds, on the other hand, are extremely durable, but they are more difficult to work with and they dry slowly. Also, solvents must be used with alkyds to clean brushes, rollers, paint trays, and drips.
One of the alkyd types of exterior paint may be especially appealing because of its regulated, self-cleaning property. It's called "chalking," and that's exactly what it does. Over a period of years, the paint surface slowly oxidizes. Each rainfall washes off a minute quantity of the paint -- along with dirt. As a result of this shedding, the paint surface is constantly renewing itself. The price of this convenience used to be chalky residue on foundations and shrubs, but the newest formulas control the shedding so it doesn't stain adjacent surfaces.
Chalking paint is not recommended for every house. In areas with little rainfall, for example, the powder tends to remain on the surface, dulling the paint. In wet regions, chalking paint may not be worth the extra expense because frequent rainfalls will keep the outside of the house clean no matter what kind of paint is used. If you live in or near either of these climatic extremes, ask your paint dealer if the chalking type is suitable for your area.