The Golden Rule: Paint Is Only as Good as What's Beneath It
Professional painters often say that a great paint job is 80% preparation and 20% application. It sounds clichéd because it's true. Paint is a thin, translucent layer that magnifies every imperfection beneath it. Scratches, pinholes, rust, and contamination all telegraph through even multiple coats of lacquer. Get the surface right and the painting becomes almost easy.
Step 1: Strip and Assess
Before any sanding begins, you need to know what you're working with. Strip the panel back as far as needed — whether that means removing old paint with chemical stripper, a media blaster, or grinding — and assess the metal underneath.
- Surface rust: Red or orange oxidation on the surface. Treatable with rust converter or mechanical removal.
- Pitting: Small craters left by rust eating into the metal. Requires filler or welding depending on depth.
- Previous filler: Old filler can hide structural issues. Probe suspicious areas and remove questionable filler entirely.
- Dents and creases: These need bodywork before any priming begins.
Step 2: Rust Removal and Treatment
Never paint over rust — even with rust-converting primer. While rust converters chemically neutralise surface oxidation, they work best when the majority of loose rust is already removed mechanically first.
- Use an angle grinder with a flap disc or wire wheel to remove all loose, flaking rust.
- Sand the area to clean, bright metal where possible using 80–120 grit.
- Apply a phosphoric acid-based rust converter to any remaining rust staining, following the product's dwell time.
- Once neutralised and dry, the surface is ready for an etch primer.
Step 3: Filling and Shaping
After metalwork, apply your body filler (polyester-based stopper for minor imperfections, two-part filler for deeper work). Mix the hardener precisely — too little and it won't cure fully; too much and it dries too fast and becomes brittle.
Block-sand filler with a long sanding board rather than a handheld pad. Boards reveal high and low spots that flexible pads follow and disguise. Work through 80 grit to shape, then 120 to refine, and finish at 180 before priming.
Step 4: Degreasing — The Most Critical Step Nobody Respects
Oil and silicone contamination cause paint to fish-eye and refuse to bond properly. Before any primer goes on, the entire surface must be degreased thoroughly.
- Use a dedicated automotive panel wipe (isopropyl-based or a purpose-made degreaser).
- Wipe with one cloth, dry with a fresh clean cloth immediately — don't let the solvent evaporate on the surface carrying contaminants back with it.
- Wear nitrile gloves — bare hands deposit skin oils instantly.
- Degrease again after any sanding, always as the final step before spraying.