PROBLEM: Cedar Stain (Tannic Acid Bleed)
CAUSE: Stains from red cedar, cypress, and redwood are due to a combination of moisture and insufficient sealing. Moisture carries water-soluble color extracts (tannic acid) contained in the wood through paint films. Staining is more likely to be visible when light or medium colors are used. It occurs more frequently with a latex coating than a solvent or oil based coating.
SOLUTION:
UNPAINTED WOOD: If staining occurs during paint application, use two coats of primer before topcoating. A waiting period of 24 hours after application of the first primer is recommended before applying the second coat and/or finish coat. Tannic acid stains will penetrate the first coat of primer leaving brownish streaks or rings. If stains do occur, reprime the stained spot, allow to dry and apply the finish coat. Sometimes it is best for the new wood to weather for several weeks prior to painting.
PREVIOUSLY PAINTED SURFACES: Stained areas can be washed with a one-to-one mixture of denatured alcohol and water. Wood bleach (oxalic acid) can also be used. Rinse treated areas well, let dry, then apply one coat of desired primer and topcoat.
PROBLEM: Wax Bleeding on Hardboard Siding
CAUSE: A cause of discoloration seen on painted hardboard siding is "wax bleed." Wax or petrolatum is used in the manufacture of hardboard siding to make it moisture resistant. Wax can migrate to the surface. Once on the surface, wax can change the appearance of a coating by its "wetting" effect or its tendency to increase dirt retention. Contributing factors include:
- Dark paints exhibit discoloration more readily due to a tendency to absorb heat.
- Areas lacking adequate coats are more likely to stain. This appears as irregular discoloration on brushed or rolled surfaces where wax bleeding occurs in valleys or thin areas in the application pattern.
- Paints containing low levels of binder are more porous and are more likely to allow wax migration from hardboard.
- Exposures subjected to direct sun and heat will accelerate wax bleed.
SOLUTION: In order to correct a situation where discoloration exists, one must first identify the problem as wax bleed. Do the following:
- Place a few drops of household bleach on the discolored area. If there is no whitening or bleaching on the surface, the stain is probably wax.
- Place water droplets on both normal and discolored areas. If the water beads up and runs off, a wax film is likely.
- Areas of discoloration should be cleaned with a detergent solution if the surface wax is light. In severe cases, surface must be thoroughly cleaned by wiping with rags wet with a solvent such as mineral spirits. The rags should be changed frequently and the surface allowed to dry before painting.
PROBLEM: Blistering
CAUSE: Blistering refers to the formation of a blister caused by vapor expanding between the coated surface and the film. Due to humidity, construction defects, or lack of effective vapor barriers, moisture condenses on or saturates wood siding. Lower humidity and high temperature generate pressures within wood and on adhesive bond of the coating as trapped moisture evaporates. Solvent based coatings are most susceptible. Fresh blisters are filled with water. As the surface dries, water evaporates, but the paint film does not recover. Blistering is accelerated by the swelling effect of moisture on wood. Continued swelling and contraction between day and night humidity and temperature variations weakens adhesion. Another cause is solvent entrapment, or painting with solvent based coatings in direct sunlight on a surface that is too warm. This problem is prevalent in darker colors, due to absorption of heat, more than light colors. Thicker application or recoating prior to complete cure of a previous coat contribute to this type of failure. Under this condition, the paint surface dries before all the solvent is released from the underlying film. Expansion of trapped solvent produces blisters.
SOLUTION: If blistering was caused by moisture, a number of solutions exist. Repair loose caulking and install vents or exhaust fans. If the home has lap siding, install siding vents. Vents or fans allow moisture from in the house to evaporate before penetrating wood siding. This will minimize future blistering problems. Sand and scrape peeling paint to bare wood. Prime exposed areas and repaint. If large areas of paint have blistered and need to be removed, high pressure washing or use of a heat gun will speed the process. Heat blisters develop in FRESHLY applied coatings only. When broken, they don't contain liquid and are not associated with a surface defect that requires remediation. These blisters may be removed by scraping and sanding. The surface should be recoated under more favorable conditions.
