CSLB C-33 Painting & DecoratingSurface PreparationMedium
A painting contractor is preparing to paint a newly installed exterior wood trim that is made of redwood. The contractor is concerned about potential tannin bleed-through. Which type of primer is most effective for preventing this issue?
- AEpoxy primer.
- BPolyvinyl acetate (PVA) primer.
- CWater-based acrylic latex primer.
- DOil-based alkyd primer.
Show answer & explanationAnswer & explanation
Correct answer: D. Oil-based alkyd primer.
Redwood, like cedar, is rich in tannins (natural wood extractives) that can bleed through water-based primers and paint, causing discoloration. An oil-based alkyd primer effectively seals these tannins, preventing bleed-through and ensuring a clean, uniform topcoat. Water-based primers are less effective for this issue, and PVA/epoxy primers are not designed for tannin blocking on wood.
Why the other options are wrong
- A. Epoxy primers are typically used for concrete or metal and are not suitable for blocking tannins on wood.
- B. PVA primers are primarily for drywall and do not have the necessary tannin-blocking properties for redwood.
- C. Water-based acrylic latex primers are generally less effective at blocking tannins and may allow them to bleed through, causing discoloration.
Tannin Bleed-Through Prevention
To prevent tannin bleed-through from woods like redwood or cedar, an oil-based alkyd primer is the most effective choice as it seals in the natural wood extractives.
- Tannins are water-soluble.
- Oil-based primers create an insoluble barrier.
- Bleed-through causes yellow/brown staining.
Memory trick: Redwood's rich runoff needs an oil-based 'off' to keep colors right.