CSLB C-33 Painting & DecoratingSurface PreparationMedium
A painting contractor is preparing a fiberglass shower stall for repainting. The existing gel-coat surface is intact but appears slightly dull. What is the most appropriate method to prepare this surface for optimal adhesion of the new coating?
- AApplying a thick coat of bonding primer directly over the dull surface.
- BLight scuff sanding with fine-grit sandpaper (e.g., 220-320 grit) followed by cleaning.
- CAggressive sanding with 60-grit sandpaper to create a deep profile.
- DWiping down with mineral spirits to remove all contaminants, then painting.
Show answer & explanationAnswer & explanation
Correct answer: B. Light scuff sanding with fine-grit sandpaper (e.g., 220-320 grit) followed by cleaning.
For an intact but dull fiberglass gel-coat, light scuff sanding with fine-grit sandpaper creates a mechanical profile for new paint adhesion without damaging the substrate. Aggressive sanding can damage the gel-coat. Wiping with mineral spirits cleans but doesn't create a profile. Applying primer without proper surface prep can lead to adhesion failure.
Why the other options are wrong
- A. Applying bonding primer directly over a dull, un-abraded surface may lead to poor adhesion and premature peeling of the new coating.
- C. Aggressive sanding with coarse grit can severely scratch and damage the gel-coat, making it difficult to achieve a smooth finish.
- D. Wiping with mineral spirits removes contaminants but does not create the necessary surface profile for optimal mechanical adhesion of the new coating.
Fiberglass Gel-Coat Prep
Preparing an intact fiberglass gel-coat for repainting involves light scuff sanding with fine-grit sandpaper to create a mechanical profile for adhesion, followed by thorough cleaning.
- Gel-coats require mechanical abrasion for adhesion.
- Fine-grit sanding prevents damage.
- Thorough cleaning after sanding is critical.
Memory trick: Dull gel-coat needs fine-grit's touch, not harsh scrapes or just a quick rush.