Home exteriors are subject to all sorts of damaging causes: rain, wind, sun's UV rays, hail, and a host of other abuses. Interior house painting, because of its controlled environment, has a high tolerance level for errors that exterior painting does not share. Because of this, you need to get your exterior paint job right, or it will begin to fail within a season or two. Understanding why exterior paint fails is the first step in understanding how to help your paint be as durable and beautiful as possible for years to come.
Wood Was Wet When It Was Painted
If only the surface of the wood is wet, then only one sunny day is usually needed for drying before painting. If the wood is saturated, several sunny or windy days are necessary.
Unfinished Siding Was Exposed to Weeks of Sunlight
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 three weeks of sunlight before painting, lightly sand or power wash the surface to remove the thin layer of degraded wood before applying paint.
Temperature Was Too Cold When Wood Was Painted
Oil-based paint should be applied when the temperature is at least 40 F; for latex paints, it should be 50 F. Conditions should remain above these temperatures for a full day and night after painting. When pre-treating the wood with a paintable water-repellent preservative (a recommended practice), best results are achieved when applied at greater than 70 F.
Wood Was Too Hot When It Was Painted
Do not paint when the temperature reaches 90 F or greater. 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 two 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.
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.
Humidity in the House Was Too High
A high level of humidity inside the house is often the cause of paint failure that occurs on the outside walls of the bathroom or kitchen. 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.
Wood Was Installed Directly Over Foam or Foil-Faced Insulation Board
Water can travel in behind the siding of the house through various routes but has to travel out through the wood, pushing the paint off. Even if the paint remains on the surface, this moisture can cause other problems. Large overhangs, proper caulking, and a 12-inch minimum ground clearance may decrease the chance of water getting in behind the siding. Additional suggestions to prevent paint failure in this situation include the following:
- Driving small wedges (1/16-inch) under every sixth row of siding may permit water to escape and reduce the moisture problem. However, wind-driven rain may also use this as access and aggravate the situation.
- Back priming (painting the back of the siding before installation) may help reduce/prevent paint failure.
- Install roofing paper beneath the siding. The best solution is to attach furring strips to the studs through the insulation board, making air spaces behind the siding. Furring strips can also attract insects if you do not screen the bottom. A spacer-type webbing called cedar breather is sometimes used under wood shingles and may have merit for use under the siding.
House Has No Interior Vapor Barrier
The absence of an interior vapor barrier is related to the problems of high levels of humidity inside the house during the heating season and wood that was installed directly over foam or foil-faced insulation board. Driving wedges under the siding may be the easiest solution. Applying certain interior vapor-retarding paints and installing electrical outlet gaskets may also be effective, especially on the upper floor of a multi-story house.
Wood Siding Is Dirty
If the siding is dirty, the surface of the siding should be power washed or cleaned with detergent and a stiff bristle or brass brush and rinsed well. Never use steel or iron, which causes iron stain and may glaze the surface.
Wood Has Mill Glaze
Mill glaze can cause the paint to peel; it can be caused by several factors. For example, if during planning to make smooth wood, the planer blades were dull, running too fast, or pressing too hard on the wood surface, the surface of the wood can become hardened, or resins may be drawn to the surface causing a glaze. Whatever the cause, the surface appears to be case hardened. If a drop of water beads up on the wood surface but does not on a lightly sanded surface, you may have what is commonly called mill glaze.
If you have mill glaze, the smooth surface must be lightly sanded or power washed to remove the hardened surface. Controlled wetting of new siding with a garden hose may promote better adhesion to the re-dried wood. The water releases the stresses in the wood. You can also create a type of mill glaze by sandblasting or using a wire wheel on the wood surface. Mill glaze is not a problem on a rough-sawn siding.
Brown Stains Appear on the Surface of the Paint
Paint does not have to fall off to fail. Moisture traveling through wood pulls water-based extractives through the paint, leaving brown stains on the surface of the paint. If the wood is kept dry, the water-based extractives in the wood will not bleed through paint. Keeping all moisture out may be difficult. Oil-based primers usually block extractive stains better than latex primers and may be a better choice on redwood and cedar; however, oil paints can increase mildew. Compared with oil-based primers, latex primers produce a more flexible paint film with better durability and can be used when extractive staining is not a problem.
Wood Has Decayed
Decayed wood can result if the wood has been wet for extended periods. If the wood is soft and spongy, it is degraded to the point that it will never hold paint and should be replaced.
Do the following to prevent house paint failure:
- Install siding properly.
- Sand or power wash the surface of the wood if it is smooth.
- Apply paint during recommended weather conditions and temperatures.
- Treat the surface of the wood with a paintable water-repellent preservative (especially the end grain).
- Prime the surface of the wood with a stain-blocking primer.
- Properly apply caulking material.
Note: This article is an adaptation from the USDA Forest Service article "Why House Paint Fails," by Mark Knaebe and uses portions of the original text. This article is available in the public domain and is found at the Forest Products Laboratory website.