CSLB C-33 Painting & DecoratingSurface PreparationMedium
A painting contractor is tasked with painting a newly constructed, unprimed cedar siding exterior. The client is concerned about tannin bleed-through, especially with a light-colored finish paint. Which type of primer is most appropriate for this application?
- AA standard latex exterior primer.
- BA PVA (Polyvinyl Acetate) primer.
- CAn oil-based or shellac-based stain-blocking primer.
- DA high-build acrylic primer.
Show answer & explanationAnswer & explanation
Correct answer: C. An oil-based or shellac-based stain-blocking primer.
Cedar and other 'bleeding' woods contain tannins that can migrate through water-based primers and topcoats, causing discoloration. Oil-based or shellac-based primers are effective stain blockers that seal in these tannins.
Why the other options are wrong
- A. Standard latex primers are water-based and generally do not block tannins effectively.
- B. PVA primers are for drywall and plaster; they do not block stains like tannins.
- D. High-build acrylic primers are for filling and leveling, not primarily for stain blocking.
Tannin Bleed-Through Prevention
The use of specific primers to prevent natural wood extracts (tannins) from migrating through paint films, causing discoloration.
- Common with cedar, redwood, and other 'bleeding' woods.
- Requires oil-based or shellac-based primers.
- Water-based primers are generally ineffective.
Memory trick: Tannins seep, oil-prime deep.