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Tannin
Staining: Brownish or tan discoloration on the paint
surface due to migration of tannins from the substrate
through the paint film. Typically occurs on "staining
woods", such as redwood, cedar and mahogany, or over
painted knots in certain other wood species.
Possible Causes:
Failure to adequately prime and seal the surface before
applying the paint.
Use of a primer that is not sufficiently stain-resistant.
Excess moisture escaping through the exterior walls, which
can carry the stain to the paint surface.
Solution:
Correct any possible sources of excess moisture
see Efflorescence and Mottling. After thoroughly cleaning
the surface, apply a high quality stain-resistant
oil-based or acrylic latex primer. Oil-based
stain-resistant primers are the best type to use on
severely staining boards. In extreme cases, a second coat
of primer can be applied after the first has dried
thoroughly. Finish with a top quality latex paint.
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