CSLB C-33 Painting & DecoratingSurface PreparationMedium
A painting contractor is preparing a newly installed, unprimed exterior cedar siding for painting. The client wants to prevent potential tannin bleed-through, which is common with cedar. Which type of primer is most effective for this specific issue?
- AWater-based acrylic primer
- BShellac-based primer
- CPVA (Polyvinyl Acetate) primer
- DOil-based stain-blocking primer
Show answer & explanationAnswer & explanation
Correct answer: D. Oil-based stain-blocking primer
Oil-based stain-blocking primers are highly effective at sealing in water-soluble tannins found in cedar and redwood, preventing them from migrating through the paint film and causing discoloration (tannin bleed-through).
Why the other options are wrong
- A. Water-based acrylic primers can reactivate and allow tannins to bleed through, especially on new cedar.
- B. While shellac-based primers are excellent stain blockers, oil-based primers are generally preferred for larger exterior applications like siding due to cost, ease of application, and good exterior durability.
- C. PVA primers are designed for drywall and do not offer adequate stain-blocking properties for tannins.
Tannin Bleed-Through Prevention
The use of specific primers to seal in natural wood tannins, preventing them from leaching through paint and causing discoloration.
- Common issue with cedar, redwood, mahogany.
- Oil-based primers are highly effective.
- Water-based primers may reactivate tannins.
Memory trick: Tannins need oil to be 'blocked' from the paint.