NASCLA Accredited Commercial General ContractorFinishesMedium
A painting contractor is preparing to paint a newly plastered wall in a historic building. To ensure proper adhesion and prevent alkali burn-through, what type of primer should be applied before the finish coats?
- AAlkali-resistant primer
- BOil-based primer
- CShellac-based primer
- DStandard latex primer
Show answer & explanationAnswer & explanation
Correct answer: A. Alkali-resistant primer
New plaster, especially traditional lime-based plaster, is highly alkaline. An alkali-resistant primer is specifically formulated to withstand the high pH of fresh plaster, preventing the topcoat from reacting with the alkali (alkali burn-through) which can cause discoloration, peeling, and poor adhesion.
Why the other options are wrong
- B. Oil-based primers can offer good stain-blocking but are not specifically designed for alkali resistance and can saponify (turn into soap) on highly alkaline surfaces.
- C. Shellac-based primers are excellent for stain and odor blocking but do not provide specific alkali resistance for new plaster.
- D. Standard latex primers are not typically formulated to resist the high alkalinity of new plaster and can fail.
Alkali-Resistant Primer
A specialized primer formulated to withstand and block the high pH (alkalinity) found in new masonry, concrete, and plaster surfaces, preventing damage to subsequent paint layers.
- Essential for new or unsealed alkaline substrates.
- Prevents efflorescence, discoloration, and adhesion failure of topcoats.
- Creates a stable surface for optimal paint performance.
Memory trick: New plaster's bite needs an alkali shield, paint's future revealed.