Surface Preparation and Substrate Assessment
What the exam tests
This domain is about identifying what you are painting and preparing it so the coating can bond. Exam items cover substrate identification, cleaning, sanding and featheredging, adhesion testing, and the primer choice that matches steel, aluminum, galvanized, plastic, or an existing finish.
Core rules
- Wash first with a detergent intended for paint prep, rinse, and dry. Then use a solvent wipe and tack cloth. Wax, silicone, and road film are not removed by sanding alone; sanding can drive contamination into scratches.
- Identify the substrate before sanding. A magnet, paint-thickness gauge, visual inspection, and a solvent or sanding test help reveal steel, aluminum, galvanized, plastic, fiberglass, e-coat, or prior repair.
- Do not sand through OEM e-coat on steel if the product system allows it. If you break through, use the primer specified for bare metal.
- Aluminum needs the system's epoxy or conversion coating; bare steel needs an epoxy or etch primer as specified; galvanized needs careful cleaning and a compatible primer, not aggressive grinding that removes the zinc.
- Plastics require cleaning, adhesion promoter where specified, and flex additive in the topcoat system when the part is flexible.
- Featheredge repairs and block sand with a grit sequence that removes scratches from the previous grit. Follow the product sheet for final grit before sealer or base.
- Check adhesion and recoat windows before building more film.
Worked example
A used door has a tight, checked finish and unknown prior repair. A solvent wipe softens the old coating and a thickness gauge shows excessive film. The correct response is to strip or sand to a stable substrate, identify bare areas, apply the specified primer, block sand, and seal. Painting over a checked, unstable film may look good for a week but fail by lifting or cracking.
Traps that cost marks
- Sanding before washing, which pushes wax and silicone into the surface.
- Using one primer for every metal.
- Grinding galvanized coating away and treating it like bare steel.
- Skipping a final solvent wipe or touching the panel with bare hands.