Defect Diagnosis and Repair
What the exam tests
This domain asks you to diagnose defects by cause, not just by appearance: runs, sags, orange peel, fish-eye, solvent pop, blushing, mottling, dieback, peeling, cracking, pinholes, dust nibs, and overspray. Expect questions that give a symptom and ask for the most likely cause and the correct repair.
Core rules
- Diagnose before sanding. Identify whether the defect is in the substrate, primer, basecoat, clearcoat, or application process. Sanding a contaminant back into the film makes it worse.
- Fish-eye is usually oil, silicone, or water contamination from the air line, shop, or surface. Correct the source, clean the gun and air supply, and use the system's additive only as directed.
- Solvent pop and pinholes come from trapped solvent or air under a skinned film, often from heavy coats, short flash, or high heat.
- Runs and sags come from too much material, too close a gun, too slow a pass, or too little flash. Wait for cure, then block sand and polish or recoat.
- Orange peel is a flow problem: viscosity, pressure, distance, temperature, or gun setup. Do not simply sand and polish a severe case; correct the application.
- Blushing is moisture in the film, common in high humidity or from a wet air line. Dieback is loss of gloss, often from over-reduction or incompatible products.
- Repair to the correct layer. If the defect is in the clear, sand and recoat clear; if it is in the base or primer, go deeper.
Worked example
A fresh clearcoat has small crater-like openings that look like fish-eye. The technician checks the air line and finds the coalescing filter overdue. The reasoning: oil aerosol from the compressor is a classic fish-eye cause. The correction is to service the air system, clean the gun, sand the affected area, and recoat. Adding fish-eye eliminator without fixing the air supply may only mask the problem.
Traps that cost marks
- Polishing a run before the film has cured.
- Confusing solvent pop with fish-eye and treating the wrong cause.
- Sanding through a defect into a incompatible layer.
- Recoating over contamination without cleaning and testing.