What causes reddish colored stains on cedar and redwood?
The culprit here is a water-soluble dye called tannin. Moisture in the wood will cause the tannin to migrate to the surface and stain the paint. New red-colored wood must be sealed with a quality alkyd or latex undercoater to create a barrier between the topcoat and the wood. More severe cases of tannin bleed may need one or two coats of an alkyd primer to prevent discoloration.
