PRIMERS AND PAINTS FOR WOOD This standard includes general information on primers and paints to be used on interior and exterior wood surfaces. The primary purpose of paint is to protect wood from deterioration. To do so, paint manufacturers have developed paint systems which are made to work together to protect the wood substrate. These systems include primers and appropriate, compatible top coats which can vary depending on the substrate and can vary between manufacturers. As a result appropriate primers and compatible top coats, both from the same manufacturer should be used. For information on paint removal from wood, surface preparation and application procedures, see the following: - For guidance on paint removal from wood, see 06400-07-R "Chemically Removing Paint From Wood Features", 06400-02-S "Supplemental Guidelines for Removing Paint from Interior and Exterior Wood Surfaces" and 06400-09-R "Removing Paint From Wood Features Using Thermal Methods". - For guidance on preparing wood surfaces for painting, see 06300-02-R. - For additional information on the history, properties and uses of paint, see 09900-01-S. - See 09900-07-S for general guidelines on painting interior and exterior surfaces. ***PRIMERS FOR WOOD*** CHARACTERISTICS - They prevent certain chemical reactions from occurring between the wood and the top coats. In wood, water soluble extractives are often a natural constituent of many wood species. Stain blocking primers, either oil- or water-based are especially important to use in these situations. - They provide a more stable substrate for the new top coats. - They provide a uniform coat allowing more even color coverage of the top coats. - They bind weathered wood fibers, providing a more stable substrate. - For areas subject to heavy mildew, mildew resistant primers are also available, making the surface less susceptible to mildew. TYPES Oil/Alkyd Primers: - Must be applied to a completely dry surface. - Recommended for use when all of the paint has NOT been removed from the surface. Oil based paint is better able to bind with old, chalky paint layers thereby providing a more stable base for the finish coats. - Recommended for use if the existing paint type is unknown (oil or latex?), or if a switch to latex top coat is being made. - Appropriate for use on wood surfaces from which all paint has been removed but which are subject to less movement, i.e. a wall surface rather than a fence. - Advantages: 1. Have excellent stain blocking capabilities. 2. Suited for wood in which the paint has NOT been removed. Acrylic Latex Primers: - Found to be successful when used on wood surfaces from which all of the paint has been removed and from wood surfaces which are "fresh". - They are especially good for surfaces which are considered difficult situations i.e. free-standing elements such as fences, columns, balustrades, etc. where wood is constantly moving. - They can be applied to wood that is slightly damp. - Advantages: 1. More flexible than oil/alkyd paints. - Limitations: 1. As latex paint dries it shrinks more than oil and can literally pull off older, more brittle paint layers as it dries. Therefore, oil/alkyd paints are usually recommended for use on wood surfaces where all of the paint has not been removed. - Products: 1. Lucite brand of Forest Products Lab: Found to be the most flexible and is the only acrylic latex primer which has a good stain blocker. 2. If Lucite is unavailable and the wood being painted contains water soluble extractives (cedar and redwood), then an oil/alkyd primer is recommended. 3. If in doubt about the stain-blocking capabilities of a selected latex primer, test it. ***PAINTS FOR WOOD*** CHARACTERISTICS - Made up of three basic ingredients: 1. A binder - oil or water 2. Thinner - mineral spirits, turpentine or water 3. Pigment - organic or inorganic - To these basic ingredients can be added any number of additives to produce specialized paints. - The term vehicle, often used in reference to paint, refers to the binder plus the thinner. TYPES NOTE: The paint selected must be from the same manufacturer and made to be used with the primer selected. It should also be selected for use in a specific situation where applicable, such as using porch and deck enamel when painting porch floors. CAUTION: PAINTS CONTAINING ZINC ARE TO BE AVOIDED WHEN PAINTING WOOD, AS ZINC ATTRACTS MOISTURE. Oil/Alkyd Paints: - Opaque coatings which use natural oils, such as linseed oil, or modern alkyds as the binder. - Alkyds are oil modified resins which dry faster and harder than ordinary oils. - They offer the best protection from both liquid and vaporous water but become brittle with age and eventually are unable to move with the substrate and peel, crack, flake, etc. - Advantages: 1. Durable. - Limitations: 1. Longer drying time. 2. More difficult to clean up than latex paints. 3. Can be odorous, volatile and flammable due to the presence of organic solvents. Emulsion or Latex Paints: - Also known as water based paints, these paints have a latex binder which has been emulsified or suspended in water. - Acrylic latex resins are particularly durable and favored over polyvinyl acetate and polyvinyl chloride latex resins. - They allow more water vapor to pass through than oil based paints and they are more flexible, even over time. Nevertheless, they will eventually peel, flake, crack, etc. - For optimal results when using acrylic latex paints, make sure that for at least the first 24 hours after application, a temperature of 500F. can be maintained. - Advantages: 1. General: a. Easy to clean-up. b. May be thinned with water. c. More flexible than oil/alkyd paints. d. Provide better resistance to mildew because there is no oil in the paint. The oil of oil/alkyd paints acts as food for mildew. NOTE: THERE HAVE BEEN CASES WHERE MILDEW HAS PROLIFERATED EVEN ON A LATEX PAINT SURFACE; APPARENTLY THE TINT USED TO COLOR THE PAINT PROVIDED THE NECESSARY FOOD SOURCE. e. Decreased odor, toxicity and flammability (due to lack of organic solvents and thinners). 2. Acrylic-based paints: a. Excellent color and gloss retention. b. Good flexibility and durability. 3. Polyvinyl acetate emulsion paints: a. Low cost. b. Excellent color retention. 4. Styrene-butadiene paints: None identified. 5. Linseed-oil resin-emulsion systems: a. Easy to make. b. Low material cost. c. Improves paint durability. - Limitations: 1. General: a. Some emulsion paints require the use of a special primer or sealer to seal chalky surfaces and prevent peeling of the new coating. 2. Acrylic-based paints: a. Sometimes have poor color retention in dark tints. b. Sometimes combined with alkyd-resins for better adhesion; This increases the potential of mildew growth. 3. Polyvinyl acetate emulsion paints: a. Moderate durability when used alone; Durability is increased when the vinyl acetate emulsion is blended with other emulsions (i.e. acrylic, linseed oil, alkyd-resin). 4. Styrene-butadiene paints: a. Normally not used on exterior. b. Tend to yellow with age. c. Not very flexible - grain cracking is a frequent problem when applied to wood. d. Not readily available today. 5. Linseed-oil resin-emulsion systems: None identified. - Products/Suppliers: 1. Benjamin Moore and Co. 2. Glidden 3. PPG Industries, Pittsburgh Paints 4. Pratt and Lambert 5. The Sherwin-Williams Company NOTE: There are some paints on the market known as self- cleaning paints. As rainwater runs down the wall the paint is slowly worn away. This is called chalking. Such paints should NOT be used if there is an unpainted surface or contrasting color below the painted surface, such as a red brick foundation wall. The streaking can be unsightly. END OF SECTION