Defects, Faults and Their Remedies
Defects, Faults and Their Remedies
This domain tests diagnosis: given a defect, identify the cause and the correct remedy. Marks are lost by naming a remedy without linking it to the cause.
Common defects
- Runs and sags: too much paint, too thin a film, or a gun held too close. Remedy: sand back, adjust application, recoat.
- Orange peel: poor atomisation, wrong thinner, or surface too warm. Remedy: sand and polish or recoat with correct viscosity.
- Blistering: moisture or solvent trapped under the film. Remedy: strip, dry the substrate, prime correctly.
- Peeling and flaking: poor preparation, grease, or applying over a powdery surface. Remedy: remove to sound edges, clean, prime.
- Chalking: binder breakdown from UV. Remedy: remove chalk, use a UV-resistant topcoat.
- Blooming or blushing: moisture condensing in a fast solvent film. Remedy: stop work in high humidity, add retarder, polish out.
- Wrinkling: too thick a coat or a hard topcoat over a soft undercoat. Remedy: strip and rebuild with thinner coats.
- Pinholing and fish eyes: solvent escape, oil or silicone contamination. Remedy: clean the surface and air line, use a compatible thinner.
Worked example
Exterior wood shows blisters that break open to reveal bare timber. The likely cause is moisture in the wood escaping through the film, not solvent entrapment, because the blisters are at the timber and appear after rain. The remedy is to strip the area, dry the wood, treat knots and prime before recoating.
Traps
- Treating a contamination fish eye by adding more paint rather than cleaning.
- Sanding a run level and recoating without allowing the coat to harden.
- Confusing blooming with chalking; both look pale but have different causes.
- Recoating over a powdery chalk surface without removal.