Application of a wood conditioner, thinned linseed oil or thinned shellac to wood prior to staining. You would apply a washcoat to wood that might blotch when stained with a liquid stain.

a coat of thinned finish that's applied to bare wood to partially seal the surface before a stain is applied

a thinned finish that will partially seal the wood but still allow the stain to penetrate

Special surface in a catalytic converter, comprising a benign substance, usually a rare metal, that acts as a catalyst.