california redwood association
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Currently, non-film-forming finishes do not provide reliable protection against extractive discoloration. Micronized paraffin wax dispersed in a solvent can provide good water repellency and retard liquid water absorption and extractive dissolution. Wax breaks down relatively quickly, however, and once the water beading has stopped, extractives could appear at the surface.
Other systems use 3 or 5 percent silicone water repellents based on silicone resins. These systems do form a film but it is not visible due to their low solids and good penetration. They usually repel water longer than wax dispersions but are normally used for masonry and are difficult to paint.
Solvent-borne oil and alkyd resin based film-forming finishes are acknowledged as being the most effective at resisting extractive staining on wood sidings with a high content of water-soluble extractives. They form a film that is less permeable and resists moisture penetration much more effectively than water based finishes, essentially sealing extractives into the wood; however, if these products are thinned excessively or coverage is inadequate, extractive staining can occur.
Water-based stain-blocking primers have been developed that are effective on species with lower extractive content and on veneered sidings. These finishes offer advantages of being environmentally acceptable due to their low VOC (volatile organic compound) content, low odor, low toxicity and water clean up. Some stain-blocking latex primers are formulated as a barrier coating to block water-soluble extractives. Other systems function by chemically tying up extractives in the prime coat, preventing further extractive migration into topcoats. With these systems, prime coats typically become discolored, but this discoloration is not transferred to the topcoats. Often, manufacturers of these products recommend that two coats of primer be applied if staining occurs, or the use of a solvent-borne oil or alkyd primer. (One system uses an acrylic emulsion polymer additive at approximately 54% by volume(5).) Many other formulations have been developed and tested. One recent study investigated the effects of dispersants, thickeners, pigments, resins and additives on the stain blocking performance of a latex primer formulation.(3) The results of the study indicated that stain blocking effectiveness improved when 325 water-ground mica, activated calcium barium phosphosilicate and a high molecular weight, low functionally, hydrophobic ammonium salt dispersant (NH4+ salt of acid copolymer) were used in the formulation. A non-film-forming cationic acrylic polymer was also identified as significantly improving stain blocking performance. The study also suggested that of the four best performing commercially available products tested, three were low in pigment volume concentration (PVC) and the remaining one used the cationic non-film-forming polymer.
Polyurethane and alkyd resin varnishes are the best finishes to use in interior rooms where humidity is high and extractive staining is possible, such as in kitchens and bathrooms. For knot bleeding, several proprietary knot sealers are available. Varnishes and white pigmented shellacs are also used as spot primers/sealers over knots.
1) The use of solvent-borne oil or alkyd based finishes has been traditionally recommended on those species of wood which are high in extractive content. Because of environmental restrictions on the solvents used in these finishes, the use of water-borne systems is expected to increase; therefore, a need exists for water-borne finishes which will prevent extractive bleeding to the same extent as that of solvent- borne oil or alkyd finishes. The primary requirements of such finishes are to prevent the penetration of water into the wood and not to have the solvent itself contribute to discoloration.
2) A penetrating primer that could chemically bind the extractives in the substrate through use of special additives would be of interest. The pigmentation should not create other stains in the topcoat because of solubility or reactivity with industrial fumes.
3) Develop a long-lasting water repellent suitable for use with stains. Alternatively, a chemical additive to semitransparent stains that would bind extractives in the wood, as in (2) above, before they discolor the finish would also be desirable.
4) Improve knot and pitch sealers.
Extractives are generally classified according to their solubility. Water soluble extractives are most troublesome because of the difficulty of keeping water out of wood that is in service. Extractive stains can appear in several forms. Diffused discoloration can result from rain or dew penetration through the finish whereas rundown discoloration typically results when water collects on an unfinished surface, such as the back of siding or unprimed ends of boards. Water-based finishes can dissolve extractives and cause discoloration as a result.
Several factors will affect the amount and type of extractive discoloration. The heartwood of certain wood species, such as western red cedar and redwood, may have high concentrations of water soluble extractives, whereas other species, like some pines and firs, may have higher concentrations of pitches and resins. The moisture content of the wood substrate also affects extractive staining. Generally, the drier the wood prior to application of the finish, the less chance for initial extractive discoloration. Dried lumber not only has fewer water soluble extractive problems than unseasoned lumber, but kiln drying tends to harden pitches and resins making them much less likely to bleed; however, introduction of moisture into the siding at any time during the service life can cause extractive staining. The type of finish also plays an important role in preventing extractive bleeding. Non-film-forming finishes like water repellents and semitransparent stains are not effective at preventing extractive discolorations. For water soluble extractives in redwood or cedar solid wood sidings, the most effective finishes are solvent-borne oil or alkyd based film-forming finishes. Water-based stain blocking primers have also been developed that are moderately successful over cedar or redwood solid wood sidings and are preferred over veneered sidings such as plywood. For nonwater soluble extractives, knot sealers and varnishes are currently used.
There are several short term tests available for determining the stain resistance of finishes. These tests involve subjecting the specimen to high humidity using the Fog Box, Blister Box or Humidity Chamber, among others. Long term tests have also been developed, which also use high humidity.
To date, the best method of preventing extractive bleeding is actually a combination of good field practice and the use of quality products. The use of dried siding and quality, moisture resistant finishes or stain-blocking primers are important preventive measures. Good building design and detailing are important as well. If extractive discoloration does occur, it can usually be removed.
Environmental concerns and aesthetic tastes are causing changes that tend to increase the likelihood of extractive bleeding. Lower VOC requirements, the desire for a more “natural” appearance and increased use of lighter colored finishes are examples of some current changes. Additional research to develop improved water based stain blocking finishes that will meet the VOC regulations is needed. It would also be of value to develop a finish system that would chemically bind extractives in the wood, without discoloring the finish itself. This would be particularly useful for light colored stains.
As long as wood continues to be used in exposed conditions, extractive bleeding will continue to be a concern. Education of builders and homeowners as well as continued research in the area of stain blocking finishes is both encouraged and recommended.
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this site last updated: April 03, 2004