How to Seal Knots in Painted Wood

Knots bleeding through painted wood are caused by tannin or resin. Found in softwoods such as cedar or redwood, tannins both help the wood resist rot and pose a problem for painters and homeowners. Tannin stains are triggered by moisture. Either the wood was damp or a little "green" when it was installed, or moisture later caused tannins to leach to the surface. In either case they are quite easy to seal. Since the knot discoloration is water soluble, use a solvent-based sealer to stop them from coming through.

Scraper
Detergent or TSP
Rags
Oil-based or tinted shallac primer

Instructions

- 1
  Run a scraper across the knots. If there's resin residue, remove it by scraping and washing with trisodium phosphate (TSP) or multi-purpose detergent. Allow it to dry thoroughly.
- 2
  Sand the wood lightly with 120- or 180-grit sandpaper. Wipe it clean with a damp rag.
- 3
  Prime the knots with tinted shellac or oil-based (alkyd) primer. If there are just a few knots to seal, use an aerosol spray can of primer. Allow it to dry according to label directions. Most fast-dry oil or shellac primers can be top coated within an hour.
- 4
  Paint the wood with two coats of alkyd or latex paint, allowing 4 to 8 hours of drying time between coats.

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