A brownish or tan discoloration on the paint surface may be due to the migration of tannins from the substrate through the paint film. This typically occurs on ‘staining woods’, such as redwood, cedar and mahogany. It can also appear over painted knots in certain other wood species, such as pine.
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.
Correct any possible sources of excess moisture (see Efflorescence and Mottling). After thoroughly cleaning the surface, apply a high-quality, stain-resistant solvent-based or acrylic water-based primer. Severely staining boards are best treated with a solvent-based stain-resistant primer. In extreme cases, a second coat of primer may be applied after the first coat has thoroughly dried. Finish with a top-quality water-based paint