Every painter will tell you they do good prep work. Almost no one in NYC actually does thorough prep work, because thorough prep is time-consuming and expensive and clients can't see it — the beauty of the finished surface hides everything that happened before the paint went on. This is exactly why prep matters so much and why you should ask about it specifically before hiring anyone.
What Good Prep Actually Involves
Protection
Before any prep or painting begins, floors should be covered with rosin paper or canvas drop cloths (not plastic, which causes paint to pool). Furniture should be moved to the center of the room and covered. Outlets and switches should be taped. Moldings and trim should be carefully taped where they meet the wall surface to be painted. This takes time. Contractors who skip it are showing you exactly what the rest of their process looks like.
Surface Repair
NYC apartment walls have character — which is a polite way of saying cracks, nail holes, dings, settled tape seams, water stains, and evidence of previous repairs. All of these need to be addressed before paint application. Hairline cracks need to be opened slightly, filled with appropriate compound, and sanded. Large cracks need fiberglass tape and multiple coats of compound. Active water damage needs to be assessed for cause before being painted over.
Skim Coating
In pre-war apartments with original plaster or in apartments with heavily textured walls, skim coating — applying a thin coat of joint compound over the entire surface and then sanding smooth — is often necessary before painting. This adds significant time and cost but produces a dramatically superior result. The difference between a well-skim-coated wall and a textured wall painted directly is obvious the moment light hits it obliquely.
Priming
The right primer for the surface and paint system is not optional. New drywall requires a PVA primer to seal the paper. Repaired areas require spot-priming before full priming. Stained walls require a stain-blocking primer. Glossy surfaces require a bonding primer or sanding before painting. Using the wrong primer (or skipping it) leads to poor adhesion, poor coverage, and bleed-through of stains.
The Cost of Good Prep vs. The Cost of Bad Prep
Good prep adds $0.50 to $2.00 per square foot to a painting project. It's also the difference between a paint job that looks excellent for 7 years and one that starts showing defects after 18 months. In a New York City apartment where repainting is expensive and disruptive, prep is the best investment in the project.