So, that's why you want to make your paint job last. Who wants to do it again? Not me.
This guide is a reprint of a well-known but highly elusive article called "Why Your House Paint Failed," from the Forest Products Research Laboratory.
1. Wood was wet when it was painted
If only the surface of the wood is wet, then 1 sunny day is usually needed for drying prior to painting. If the wood is saturated, several sunny or windy days are necessary.
2. Unfinished siding was exposed to several weeks of sunlight before painting
Sunlight degrades the unfinished wood surface, thus it will never hold paint as well as fresh wood. If the unfinished wood is exposed more than 3 to 4 weeks, lightly sand or power wash the surface to remove the thin layer of degraded wood before applying paint.
3. The temperature was too cold when wood was painted.
Oil-based paint should be applied when the temperature is at least 40ºF; for latex paints, it should be 50ºF. Conditions should remain above these temperatures for 24 hours after painting. When pretreating the wood with a paintable water-repellent preservative (a recommended practice), best results are achieved when applied at >70ºF.
4. Wood was too hot when it was painted or was heated soon after painting.
Do not paint when the temperature is 90ºF. To prevent temperature blisters, avoid painting surfaces that will soon be heated. The best procedure is to "follow the sun around the house." The east side of the building should be painted late in the morning, the south side in the middle of the afternoon, the west side late in the afternoon.
The north side can be painted at any time during the day. However, at least 2 hours are needed for fresh paint to dry before the weather cools to the point where dew forms. If blistering on the wood surface does occur, allow the paint to dry for a few days, scrape off the blisters, smooth the edges with sandpaper, and paint the area.
5. The weather was too humid when the surface was painted.
When water-based paints cure, the water should evaporate as fast or faster than the solvents. After the water has evaporated, the paint will shrink to nearly its final shape. As the solvents evaporate, the paint chemically reacts to form a hard material. When it is too humid, water cannot evaporate and the solvents may evaporate first, causing the paint to cure while still in a water-filled state. You cannot recover from this type of disaster. Oil-based paints will also fail if weather conditions are too humid.
6. Humidity in the house was too high during the heating season.
A high level of humidity inside the house is probably the cause if paint failure occurs on the outside walls of the bathroom or kitchen, and it can be even more pronounced on the outside of an upper floor.
In multi-story buildings, there is a chimney effect. Warm moist air is trying to vent upstairs, and eventually this moisture travels out through the siding. Paint failure may be more noticeable near electrical outlets or other breaks in the vapor barrier. Drier air enters the house through cracks on the main level; therefore, paint failure caused by high humidity is usually not a problem on the main level. Condensation on the windows also indicates excessive humidity in the house. Turning down your humidifier or turning on a bathroom exhaust fan will help lower the humidity level inside the house. An energy efficient but somewhat expensive solution to high levels of humidity is to install an air-to-air heat exchanger. Here, warm moist air gives its heat to the incoming fresh, dry air.