As much as we want our painted surface to last forever, paint does crack, flake, peel, and blister.
Water-based or latex paints that are made with 100 percent acrylic resin (not a vinyl or vinyl-acrylic blend) have the greatest “elasticity”: The flexibility to expand and contract with surface and temperature changes. Latex paint also “breathes.” This allows moisture to escape through the latex paint. Oil-based paint forms a moisture resistant film with little flexibility that is more likely to deteriorate.
Cross-grain cracking of paint occurs when the surface coating becomes too heavy and loses flexibility. Too many coats of paint cause paint to fail on older homes, and paint applied too thickly can also crack on new structures.
Tests show that an entire paint system (primer and two coats of paint) has maximum adhesion at a thickness of six mils, or about the thickness of a sheet of newspaper.
Paint weathers — the paint film erodes — from rain, snow, and sun exposure. However, paint on the north side of most homes, on porch ceilings, and under eaves — protected areas — does not weather as much. Thus, when these areas are repainted on the same schedule as the exposed sides of the house, the paint film increases to the point that it loses elasticity. It can’t shrink and stretch with the temperature changes or with the structure’s expansion and contraction, and the paint cracks.
To prevent cross-grain cracking, paint as needed and follow the spread rates listed on the paint can. If it has occurred, completely remove the old paint, prime, and re-paint.
Painting Weathered Wood
Unless weathered wood is adequately prepared, paint cannot form a lasting bond. The gray, grainy texture of weathered wood is really a group of loose cellulose fibers. Paint applied to this unstable area has no solid surface to adhere to and will flake off as the fibers continue to deteriorate from either the weight of the paint itself or from moisture trying to escape from the interior condensation.
Use a wire brush and sandpaper to remove the loose fibers and get back to stable wood. An oil primer will sink into the paint and condition the wood before applying latex or oil-based paint to the sound surface.
Primers are formulated to give the topcoat better adhesion. Latex paint will not stick to a surface previously painted with oil-based paint. Prime the surface first with an oil-based primer and then coat with oil- or water-based paint. If you are painting over a painted surface and are unsure if the previous paint was oil or latex, rub the dry surface with a clean rag dipped in rubbing alcohol. The color from dry latex-based paint film will be picked up by the alcohol-dipped rag; oil paint will be unaffected.
• For vinyl siding always use a 100 percent acrylic latex primer and topcoat with the same color or a lighter color. Painting vinyl siding with a darker color will cause the vinyl siding to warp from the heat build-up and the paint film to crack and peel.
• For aluminum siding use an oil/alkyd primer and 100 percent acrylic latex topcoat.
• For unpainted wood use an oil/alkyd primer because a latex primer will raise the wood grain and make the topcoat fail. Stain-stopping oil/alkyd primers will also seal knotholes in wood and are used with cedar, redwood, and plywood whose water-soluble dyes will leach through water-based paint or primer. With the wrong primer the paint will flake away around the knothole.
• For new drywall or plaster, a polyvinyl acetate water-based primer works but will not block any stains. Choose an oil-based primer to hide rust and a latex stain blocking acrylic primer to hide solvent-borne stains like crayon and grease that would prevent paint from sticking adequately.
Paint must be applied within two weeks of the prime coat. After two weeks, soap-like chalky compounds form on the primed surface and will prevent the paint from adhering. If several weeks have passed since the primer was applied, scrub the surface with detergent, rinse, and then apply the topcoat.
Mildew is a microscopic fungus that looks like dirt, and when it is visible on a painted surface, it is actually eroding the paint film.
Mildew can also pop paint off a surface before the mildew spores can even be seen.
If a surface is primed in the fall and a topcoat not applied until the following painting season, mildew spores can form on the surface during winter. Come late spring, with warmer temperatures and a fresh coat of paint, the mildew has the warmth, food, and moisture needed to grow. The mildew “blooms” and actually pushes the paint off the primer.
Before repainting, wear gloves and eye protection and scrub the area with one part bleach (five percent hypochlorite solution) to three parts water. Do not mix with ammonia or any cleaners containing ammonia. Rinse with water and let dry before applying paint.
When paint is loosing adhesion, painting over it will actually lift it off. Drying paint shrinks, causing enough tension to slough off an old painted surface. It is fine to simply paint over a sound surface that has paint on it, but first make sure the paint is really as firmly attached as it looks. Take a piece of duct tape or an adhesive bandage and rub it firmly on the old painted surface. Rip the tape off. If paint flakes larger than microscopic size come off, the old paint is not firmly attached and must be removed, and the surface should be primed before a new topcoat is applied.
The average respiration produced by a family of four coupled with their cooking, dishwashing, clothes drying, and bathing can contribute three gallons of water vapor per day to a home’s humidity. Other sources of water inside the house include plumbing leaks and misdirected shower spray. If the interior side of all exterior walls does not have a vapor barrier, this moisture will permeate through the walls to escape to the outside. Blistering and peeling paint will be pronounced around bathrooms, laundry rooms, kitchens, and window frames.
Paint blistering and peeling caused by moisture will affect all the coats of paint and primer down to the bare wood surface.
Before repainting, the moisture problem must be solved with dehumidifiers, exhaust vents, or leak repairs, as needed. Vapor barriers can be installed in homes built without them by placing the vapor barrier behind new paneling or dry wall.
Bubble-like swellings can occur on the surface of the paint film as it dries. These affect only the surface coating and develop in the first 24 hours after painting. They are caused by the surface drying rapidly from being in direct sun or high wind. Think of it as a paint skin forming over wet paint. Because the temperature has risen, vapor in the wet paint expands and pushes the dry surface skin upward, hence the bubble. Temperature bubbles occur more frequently in dark colors that absorb more heat and in thickly applied coats of paint that take longer to dry.
Temperature blisters can be avoided by “following the sun around the house” so that painting is done on the north side early in the morning, followed by painting the east side at noon, then the south side and the west side late in the day. At least two hours must pass for the paint to set before the dew point (condensation) arrives toward evening.
If a surface has temperature blisters, wait several days, scrape off the blisters, feather the edges smooth with sandpaper, prime (if necessary), and repaint.
The binder in latex paint — usually acrylic or vinyl resins — is made of microscopic particles of polymer that need to fuse together to form a continuous coating. When the paint is applied below the optimum surface temperature listed on the paint can, polymers loose flexibility and do not fuse into a durable tight film. Likewise, painting at a higher than optimum surface temperature will cause the water in latex paint to evaporate before the resins have had time to adhere to the surface. Without a tight continuous paint film, the paint will peel. Check the label; most latex paints are formulated to be applied at a surface temperature between 50º and 90º F.
On flat surfaces like windowsills where water can collect or on the edges and ends of exterior wallboards where wind can drive rain in, paint will often pucker from moisture penetration.
Proper construction would have given the windowsills a slight slant and provided a wide roof overhang to protect the wallboards. Without proper construction, all that can be done is to keep on top of the problem and remove the wrinkled paint, prime, and repaint with a durable latex enamel. Paint can also wrinkle if it is applied a wet surface or if it rains within 48 hours of application.
On any paint aisle, paint labels flaunt five-year, 10-year, 15-year, and lifetime warranties. They explain how if the product fails to perform, more paint or a refund of the paint purchase price is guaranteed.
The catch is that the paint has to be “used according to label directions.” And all labels go over the basics: clean, dry, primed interior or exterior surface, optimum temperature, and spread rate. If the paint fails to live up to the warranty, it is usually not the paint but the wrong paint sheen for the surface, inadequate prep work, or poor application at fault.
Paulette Rossi is a Certified Master Recycler and freelance writer living in Portland, OR.
For more information on MetroPaint call 503-234-3000 or visit metro-region.org/paint.