Why primer and why two coats — the exterior paint film from the substrate up

What this diagram shows

Two magnified cross-sections of an exterior paint film on bare wood. On the left, a topcoat brushed straight onto bare wood wicks down into the open pores, leaving a thin, blotchy film that drinks paint, and tannin stains bleed up through it. On the right, the correct build-up: a primer coat first seals the pores, blocks stains, and bonds to slick metal or vinyl, and then two thin color coats hide evenly. Two thin coats cover far better than one thick coat, and bare or porous surfaces always need the primer.