Pratt & Lambert Paints Special Painting Problems and Solutions STAINING AND DISCOLORATION OF EXTERIOR PAINTS Exterior paints on wood, masonry and other surfaces may be stained or discolored by the oxidation of metal, corrosive fumes, mildew or by bleeding of soluble colored substances. Removal of the stains can be achieved by the following methods: Copper Stains. First, wet surface with clear water; then sponge with a solution made by dissolving 1-1/2 to 2-1/2 oz. sodium metasilicate in a gallon of water. Rub lightly with a sponge until stain disappears. Wash again with clear water. Rust Stains. Follow the same procedure as above, but use a solution of 2% oxalic acid in water or 5% phosphoric acid in water. Staining may also be prevented by painting exposed metal with a suitable exterior primer and paint. CAUTION: Oxalic Acid will cause severe eye and skin bums. Use in well ventilated areas. Wear appropriate protective equipment (e.g. safety goggles, neoprene rubber gloves and boots). Consult supplier’s material safety data sheet. Gas Discoloration. Gas discoloration is caused by the action of atmospheric sulfur gases upon lead compounds (white lead, chrome green, chrome yellow, etc.) in certain paints. These gases are common around chemical plants and oil refineries. They are also produced by the burning of soft coal, decomposition of sewage or by river and harbor pollution. Unlike metal staining, gas discoloration is a surface condition. If staining is caused by temporary contamination of the atmosphere, it will disappear as the paint film chalks. Existing discoloration can be removed or reduced by sponging the painted surface with hydrogen peroxide or a diluted solution of acetic acid, followed by a clear water rinse. Mildew. Mildew or mold is a minute plant organism of a parasitic nature which grows on surfaces supplying food (organic material such as wood, certain paints, textiles, paper). Warm, dark and moist conditions are favorable for growth of mildew. Mildew is often confused with other stains such as gas discoloration, dirt or dust. It is usually recognizable by the following characteristics:
Complete cleaning of the surface is needed before painting. Bare or painted wood should be scrubbed with the following solution:
CAUTION: Household bleach may cause eye and skin irritation. Use in well ventilated areas. Wear appropriate protective equipment (e.g. safety goggles, neoprene rubber gloves and boots). Bleeding. Bleeding is a discoloration resulting from the extraction of soluble substances from the substrate prior to painting. This discoloration can appear at once or shortly afterward. Spot prime knots and the immediate area surrounding the water sensitive area with SuPrime Int/Ext Odorless Stain Blocker. Spot prime solvent sensitive areas with SuPrime Int/Ext 100% Acrylic Multi-Purpose Primer. DISCOLORATION OVER HARDBOARD SIDING Pressboard, hardboard, Insulite and Masonite are all terms used to describe exterior siding, usually wood, which is produced under heat and pressure. During the manufacture of these materials, waxes are added to the pulp mix to resist water penetration. The wax can melt and then work its way to the surface creating a “wax bleed.” When the wax works its way to the surface, a darkened discoloration appears that can collect dirt. The wax may even work its way up through several coats of paint showing itself on even third and fourth repaints. The surface must be painted with pastel colors. Dark colors will absorb heat and draw the wax to the surface. To identify the problem the following should be done:
There is no foolproof method of painting hardboard. An alkyd primer, such as SuPrime Exterior Alkyd Wood Primer is recommended to help seal the surface. CAUTION: Household bleach may cause eye and skin irritation. Use in well ventilated areas. Wear appropriate protective equipment (e.g. safety goggles, neoprene rubber gloves and boots). PAINTING ACOUSTICAL MATERIALS The paintability of acoustical materials varies widely and depends upon the surface appearance and structure of the material. Acoustical products are made from a great variety of substances, which are all extremely porous. To maintain the acoustical value of the tiles, it is important to not close up the perforations and fissures in the tiles. Some types can be painted without impairing the sound absorbing capacity, but others must be painted sparingly so as not to lose their acoustical efficiency. The pores in close textured acoustical materials can be filled or bridged by paint, thus reducing the sound absorbing characteristics. Flat latex paint is preferred to eggshell or gloss enamel for painting acoustical surfaces because (1) it is more porous and has better acoustical efficiency; (2) it has low binder content, consequently the least tendency to bridge holes and fissures. The preferred method for coating acoustical materials is by spray application. PAINTING CONVECTORS AND GRILLES Modern decoration requires heating units and enclosures to be paintable in order to harmonize with room decor. It is important that heating units and enclosures be properly coated to avoid finish cracking or blistering and a marked change of color due to excessive heat. New metal heating units, convectors and grilles should be primed with a metal primer and topcoated with a finish coat. Make certain that the paint system is rated to withstand the heat. Under no circumstances should the heat be turned on full until the finish has thoroughly hardened. PAINTING GALVANIZED STEEL SURFACES What is galvanizing? Galvanizing is simply the application of a coating of
zinc to steel. The zinc is actually bonded (metallurgically) with the steel. These
metals are typically used for gutters, downspouts, siding, guardrails, roofs,
vents and ducts. Exposure of galvanized surfaces to natural weathering will cause
the zinc surface to oxidize. This produces a light colored film known as “white
rust.” Painting any surface is done to improve its appearance, protect the
substrate or both. Applying paint to galvanized steel is no different. Galvanized,
however, is a substrate on which it is difficult to obtain consistent results.
Failures in painting galvanized steel can be minimized if the correct steps are
taken. Essentially, there are two keys to a more successful paint job. The first
is to avoid certain pitfalls, which may lead to failure. Pitfalls To Avoid. Unprepared surfaces: Avoid painting over smooth, hard and unprepared galvanized surfaces. These lack surface texture or profiles for proper adhesion. Also, make certain all surface contaminants, oil and grease are removed. Assuming All Galvanized Are Equal: The manufacturers of galvanized metals produce numerous grades. Some are paint-ready, while others are not. Paint-ready, or bonderized, galvanized steel is treated with a chemical coating that improves the adhesion of the paint to the substrate. Spangle size also affects adhesion. The smaller the spangle size, the greater the surface texture, and therefore the better the adhesion. Ask your supplier to find out the type from the manufacturer. Chromate Treatments: Painting over galvanized surfaces that have been coated with chromate treatments, which prevent white rust, should be avoided. If this treatment is present, poor adhesion or delamination may result. Extreme Temperature Fluctuations: Painting galvanized steel in extreme temperature fluctuations can result in the cracking of a paint film. This is due to the difference in the coefficients of friction between the substrate and the coating. Painting larger areas accentuates this problem. These areas are more apt to have coating failures. Acid Cleaning Solutions: Galvanized steel is often washed with vinegar solutions or other acids prior to painting. This procedure is not recommended and can result in premature paint failures. Alkyd Primers: Alkyd Primers are not recommended for zinc surfaces. The reaction of zinc and fatty acids in the paint may produce zinc soaps that, in time, will lead to paint-metal interfacial adhesion loss. Rusty Galvanized Steel: In time, the surface may lose that galvanization and start to corrode. Prime the steel with Tech-gard Acrylic Prime or Finish or spot prime the rusted spot only with SuPrime Alkyd Metal Primer. If SuPrime Alkyd Metal Primer is applied to the intact bare galvanized, peeling will occur. Treat these areas as you would any normal ferrous metal surface. |