CSLB C-33 Painting & DecoratingCoating ApplicationMedium
A painting contractor is applying a semi-transparent oil-based stain to exterior cedar siding using a brush. After completing one section, they notice significant lap marks and uneven color. Which of the following is the most likely cause of this issue?
- AThe stain was allowed to dry too much between sections, creating visible overlaps.
- BThe cedar siding was not properly sanded, leading to inconsistent absorption.
- CThe stain was applied too thinly, preventing proper penetration.
- DThe brush used was too small for the area, causing slow application.
Show answer & explanationAnswer & explanation
Correct answer: A. The stain was allowed to dry too much between sections, creating visible overlaps.
Lap marks with stains, especially semi-transparent oil-based ones, commonly occur when wet edges are not maintained. If one section dries before the adjacent section is applied and blended, the overlap area will show a darker, distinct line due to double application and uneven penetration.
Why the other options are wrong
- B. Improper sanding could lead to uneven absorption but usually manifests as blotchiness rather than distinct lap lines.
- C. Applying too thinly typically results in lighter color or poor protection, not distinct lap marks.
- D. A small brush might slow down the process, but it wouldn't directly cause lap marks unless it prevented maintaining a wet edge.
Lap Marks (Stain Application)
Visible lines or areas of darker color where sections of stain overlap, typically occurring when a wet edge is not maintained during application, causing dried stain to be recoated.
- Common with semi-transparent and opaque stains.
- Prevented by working in manageable sections and maintaining a wet edge.
- More noticeable on large, flat surfaces and with fast-drying products.
Memory trick: Lap marks appear when the wet edge disappears.