CSLB C-33 Painting & DecoratingSurface PreparationMedium
A client requests repainting of a previously painted, high-traffic hallway with oil-based semi-gloss paint. The existing paint is in good condition but very glossy. To ensure proper adhesion of the new water-based topcoat, what is the most critical surface preparation step?
- AWashing the surface with a strong degreaser.
- BStripping all existing paint down to the bare substrate.
- CApplying a bonding primer directly over the existing paint.
- DLightly sanding (scuff sanding) the entire surface.
Show answer & explanationAnswer & explanation
Correct answer: D. Lightly sanding (scuff sanding) the entire surface.
Scuff sanding (lightly sanding) a glossy surface creates a mechanical profile, or 'tooth', for the new paint to adhere to. This is crucial for ensuring proper adhesion, especially when applying a water-based paint over a glossy oil-based finish.
Why the other options are wrong
- A. Washing removes contaminants but does not address the gloss, which prevents good adhesion.
- B. Stripping paint is a last resort for failing paint or complete substrate change, not necessary for a good condition, glossy surface.
- C. While a bonding primer helps, it performs best on a properly prepared (sanded) surface; primer alone may not adequately overcome the gloss.
Scuff Sanding
Lightly abrading a glossy or smooth surface with fine-grit sandpaper to create a 'tooth' or mechanical profile for improved adhesion of new coatings.
- Dulls the surface without removing existing coating.
- Essential for painting over glossy finishes.
- Helps ensure proper intercoat adhesion.
Memory trick: Glossy surfaces need a 'scuff' to stick.