The primary requirements of such finishes are to prevent the pen e tra tion of water into the wood and not to have the solvent itself contribute to discoloration. 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 in dustrial fumes. 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.