Why prep determines the outcome

Paint applied over a dirty, glossy, or damaged surface fails at the substrate rather than wearing evenly. Peeling, bubbling, and uneven sheen are almost always prep failures, not paint failures. The same paint applied to a properly prepared surface lasts years longer.

Cleaning and deglossing

Grease, soap residue, and airborne cooking oils accumulate on walls — especially in kitchens and near HVAC returns. Washing with a TSP substitute or degreaser before sanding removes the contamination that prevents adhesion. Glossy surfaces from existing paint or factory primer require light scuff sanding so the new coat has something to grip.

Patching and priming

Nail holes filled with spackle shrink as they dry. Skim a second thin coat after the first has dried completely before sanding flush. Larger repairs — areas wider than an inch — benefit from setting compound, which is harder and sands smoother than lightweight spackling. Spot prime any patched areas before the finish coat; bare spackle absorbs paint differently than surrounding wall and telegraphs through even two finish coats.

The primer question

Full-coat primer is worth the step when painting over a dark color, covering stains, or painting new drywall for the first time. For a repaint in a similar color range on a clean, sound surface, a quality finish paint in two coats outperforms a single coat over cut-rate primer. The specific situation determines the right approach — using primer as a substitute for prep does not work.