Brownish or tan discoloration on the paint surface due to migration or 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.
> 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
Correct any possible sources of excess moisture. 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.
Source: Paint Quality Institute