Tannin Staining is the brownish or tan discoloration on a painted surface due to migration of tannins from the substrate through the paint film. Typically, this occurs on 'tannin rich timbers, such as redwood, cedar and mahogany, or over painted knots in certain other types of timber.
Causes of tanning staining include:
- Using a coating that is not sufficiently stain-resistant
- Excess moisture escaping through the exterior timber substrate and transferring tannin stain to the surface
First, correct any possible sources of excess moisture (see Efflorescence and Mottling). After thoroughly cleaning the surface, apply an exterior top coat with tannin blocking properties. Dulux Weathershield is recommended following application instructions on the can. For severely stained boards oil based primer such as Dulux 1 Step Oil Based Primer, Sealer and Undercoat is recommended before applying the top coat.
Paint application substrate must return well under 10% moisture content; any moisture access must be isolated. Trapped moisture within the timber substrate will compromise new paint system performance and result in unsightly tannin staining.
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