How Stop Bleeding in Interior Rough Cedar

Why Stains Occur

• If you notice discoloration on the cedar's surface, and the problem isn't mold or dirt, there are two possible reasons. One is that there is enough moisture in the air or the environment around the wood to dissolve the phenolics in the wood, which then migrate to the surface with the moisture. The other possibility is that the discoloration is the result of iron staining from metal fasteners, including nails, screws, hooks and staples. You can avoid rust stains by using galvanized or enamel-coated fasteners, but if you live in a humid climate, the only way to avoid moisture stains is to seal the wood.

Clear Finishing

• If you want to preserve the natural appearance of interior rough cedar while still protecting it from stains, you need a thin finish material that isn't water soluble. Shellac is soluble in alcohol, and woodworkers have used it for centuries to provide a thin, underwhelming surface coating. It works best if you coat both sides of the cedar, but even if you just coat the top side, it will prevent moisture from migrating to the surface and bringing the stain-causing phenolics with it. You don't need a thick shellac mixture to get good stain-blocking.

Mixing and Applying Shellac

• Shellac comes in solid form, and you prepare it for use by mixing it with denatured alcohol. A 2-pound cut, made by adding 2 pounds of shellac to a gallon of alcohol, is suitable to many finishing projects, but for sealing cedar, a lighter 1 or even 1/2-pound cut suffices. It's best to spray the mixture onto rough cedar with air spray equipment because it's hard for a paintbrush to get material into all the surface crevices. Be sure to wash off existing stains with lacquer thinner before you seal. It dissolves the tannins that cause stains and removes oils from the surface so that the shellac forms a better seal.

Sealing for Paint

• If you plan on painting the cedar, it's even more important to seal it because the phenolics and tannins will leach through most paint surfaces, and the stains they create are even more visible. Clear shellac is a good prepainting sealer, but to avoid having to make a shellac mixture, use a shellac-based primer. You can find one in any paint or hardware store, usually marketed as a stain-blocking primer. Apply it by brushing, spraying or rolling. If you use a brush or roller, you made need to apply two coats to get complete coverage and protection.