Zinsser® BIN Shellac-Base Interior and Spot Exterior Primer, Interior, Flat White, 1 gal Bucket/Pail, 4/Carton

As the original shellac primer/sealer, nearly nothing is better at sealing porous materials while permanently covering and blocking wood knots, sap streaks, graffiti and a wide variety of stains as well as almost every kind of odor. Though designed specifically for indoor application, it's also capable of spot priming certain exterior surfaces (see product packaging for more details).