Clear wood is too pricey for exterior trim on all but the highest-end houses. But if you use lower grades of lumber, knots in the wood can add their own kind of extra cost. Softwoods like redwood, cedar, and even pine have sap full of pigments (the technical term is “extractives”) that can bleed through primer and paint in a process called “extractive bleeding.”
Oil-based primers and paints form a film that can block the water-soluble extractives from leaving the wood. But these days, oil-based paint is on the way out because the solvents used to mix the paint contribute to air pollution. Waterborne acrylic latex paints perform well, but they don’t form the same kind of film as the old oil paints they’re replacing—and the water they contain actually helps to dissolve the pigments in the sappy knots in the first place. So even a good stain-blocking acrylic primer may allow some unsightly telegraphing of knotholes through a paint job.
On interior wood, painters use alcohol-based shellac to pre-seal knots before they prime. Shellac blocks both oils and water for a good stain-proof seal, while also promoting adhesion between coats. But shellac on its own doesn’t stand up to outdoor conditions, where wood moves in response to the seasons. For exterior work, professional painters have learned to “sandwich” multiple coats of shellac between the stain-blocking prime coat and the paint topcoat. That way, the shellac film can do its sealing job while protected and held in place by the more flexible coats of acrylic primer and paint.