Primer
A bonding undercoat that seals the surface and helps topcoat paint stick, hide, and last.
Primer is a preparatory coat applied before your color. It does three jobs: it **seals porous or patched surfaces** so they don't soak up your finish paint unevenly, it gives the topcoat something to **bond** to, and it **blocks stains and old colors** from bleeding through. You generally need it on bare drywall or wood, over big repairs and patches, when covering stains (water marks, smoke, marker), and when making a dramatic color change such as light over dark. 'Paint-and-primer-in-one' products combine the two and are fine for repaints in similar colors, but a dedicated primer still wins on raw, stained, or drastically different surfaces. Primer counts as a coat when you estimate paint — see the [paint calculator](/guides/how-much-paint-do-i-need-calculator).