Knot bleeding

 

Brownish spotting through a paint or opaque stain where there is a knot.

Brownish spotting through a paint or opaque stain where there is a knot.

Causes

  1. This problem usually affects woods with knots containing resin (pine, spruce and fir). Solvents in coatings dissolve resin. During the drying process, solvents evaporate through the paint film giving it a brownish colouration. Knot bleeding is not the same as resinous exudation which occurs under heat or sun. In such case, the resin oozes through the coating giving it a yellowish colouration after a few months.

Solutions

  1. Seal bare wood knots with a shellac. Apply a solvent-based primer-sealer, GoPrime 190-135 or Polyprep 145-022 and a new coat of opaque coating.
  • Resinous exudation cannot be stopped. It ceases only after the knots contain no more sap. Remove resin by heating knots and scraping off excess resin. Then remove any residues with wood alcohol.
  • Do not apply shellac on knots before applying a transparent or semi-transparent coating.