PROBLEM: Lap Marks
CAUSE: Lap marks sometimes occur when wet and dry layers overlap during application of paint or stain. This nonuniform appearance can be caused by: working too long in one area when painting; too much heat or draft during application; too porous a surface; or the use of an improper thinner. Too much heat or draft causes a rapid loss of solvent or water and speeds up the set time, resulting in a thicker film wherever overlapping occurs.
SOLUTION: Another coat of paint, spread uniformly, is necessary to cover lap marks. If the finish coat is relatively transparent or the surface is overly porous, a second coat or a primer coat may be necessary. To avoid lap marks, do not paint one section of a large area from top to bottom completely. Instead, paint in small enough sections to maintain a wet edge. (Top to bottom painting is appropriate on shingle surfaces since they provide natural breaking points.) Painting on hot, windy days accelerates drying time. Avoid painting under these conditions. Add thinner sparingly if it is needed.
PROBLEM: Surfactant Leaching
CAUSE: All latex paints contain detergent-like materials called surfactants which are necessary for the stability of the paint formulation. In cases where surfactant is leaching from the dried film, this material will dissipate in small amounts. They tend to come out of the film slowly and are easily removed by normal weathering. Under certain curing conditions, such as low temperatures, condensing moisture, etc., this leaching process occurs rapidly and results in a surfactant build-up on the surface. Leached surfactant can appear as a thick brown syrup-like deposit or rundown. On occasion it may assume a white crystalline form. Surfactant leaching is not limited to exterior coatings - it may appear in bathrooms or other areas where moisture condenses on walls. In these instances, a clear amber glossy rundown will be visible.
SOLUTION: Most often, weathering removes the visible film of surfactant from the surface. The sheen and the color are restored. However, most accumulations of surfactant are observed in areas protected from natural weathering. In these instances, it can be removed with a fine mist from a garden hose or by light rubbing with a wet cloth. Often the best solution is to do nothing and let nature take its course. The surfactant will do no harm and time will correct the problem.
PROBLEM: Mildew
CAUSE: Mildew is NOT caused by paint. It is a fungus that grows on many surfaces including paint. Mildew discoloration is prevalent in warm, moist exposures, but can occur in most climates. Mold and mildew spores are microscopic particles transported by the wind. Spores can remain dormant on a surface until conditions are favorable for growth. Mildew-contaminated surfaces in neighborhoods can supply spores for contamination of other homes in the area. Mildew usually looks similar to dirt accumulation. However, on some surfaces it may appear as dark spots, or be concentrated in clusters. Under magnification a web like pattern is visible. Mildew is usually black or gray in color, but can also be dull red or yellow. To determine if mildew is present, drop a small amount of bleach on discolored surface. If mildew is present, bleach will destroy it and lighten the spots. If discoloration is simply dirt, bleach will have no effect.
SOLUTION: Mildew MUST be removed before painting. Painting over mildew guarantees its return. Mildew will grow through paint films rapidly, and correction will then require removal of the coating! All mildew-stained surfaces should be washed with a solution of 1 or 2 cups of household bleach per gallon of warm water shortly before painting. Commercial mixtures for washing the surface are also available. Repeated applications may be necessary to completely remove discoloration. Repaint as soon as the surface is dry with a mildew-resistant paint. Multiple coats of quality products resist mildew growth longer than single coats. While there is no absolute cure for mildew, quality products will contain mildew resisting properties that can minimize its growth. Periodic examination and treatment of painted surfaces, in sound condition, with mildewcide wash will restore a freshly applied appearance to the film. Due to their composition, latex products are less likely to grow mildew than oil-based products.
