Teak Conference Table Finish Crawling
A shop is finishing a solid teak conference table top, 3200 mm by 1200 mm, built from 60 mm wide strips. Some strips have dark, oily streaks. The surface was sanded to 180 grit, wiped with lacquer thinner, and left for 10 minutes before a solventborne two-pack polyurethane was sprayed directly onto the wood. Shop conditions were 20 °C and 55% RH, and the dew point was about 11 °C. The first coat showed circular depressions and pulled-back edges over roughly 15–20% of the area, concentrated over the dark streaks. The operator scuff-sanded and applied a second coat, which made the crawling worse. The client has rejected the top, and the shop has another table to make from the same teak bundle.
1. Diagnose the defect and explain why the second coat made it worse.
Read the worked answer
The circular depressions and pulled-back edges are fish eyes and crawling, not orange peel. They indicate a surface-energy problem: the coating cannot wet the wood uniformly. Teak contains oily extractives, and the dark streaks are likely richer in them. Wiping with lacquer thinner dissolves the oil and spreads it over the surface; a 10-minute flash-off can leave a thin oily film rather than removing it. The second coat made it worse because it was applied over a contaminated, partially sealed surface and added more film tension; scuff-sanding may also have burnished oil into the pores. The dew point of 11 °C at 20 °C shop temperature gives a surface-temperature margin, but if the table was cooler than the air, condensation could also contribute. The pattern concentrated over dark streaks points to extractives as the primary cause, not generic silicone contamination.
2. Propose a surface-prep and test protocol before the next top is finished.
Read the worked answer
Scrape or sand a test area to fresh wood, then wash with a fresh solvent such as acetone or MEK using clean white rags, changing rags frequently; do not flood the surface and wipe it dry with the same rag. Allow the solvent to flash off, then seal immediately with a dewaxed shellac or a manufacturer-approved barrier sealer. Run a small test panel from the same bundle, including a dark streak, through the full system. Inspect for crawling after the first coat; if it appears, do not add coats. Also check the compressed-air supply with a blotter test and confirm the table surface is at least 3 °C above the dew point. Only after the test panel passes should the production top be sprayed. If solvent washing alone does not work, try a two-pack epoxy sealer as a barrier, but verify adhesion.
3. Specify a finish schedule and acceptance criteria for the replacement top.
Read the worked answer
After the barrier sealer, apply a solventborne two-pack polyurethane at the supplier's wet-film thickness, typically two coats with a light 320–400 grit scuff between coats after full cure. Keep the shop at 20–25 °C and RH below 65%, and maintain surface temperature above dew point. Do not overcoat a crawling coat. Acceptance: no fish eyes or crawling under raking light, uniform sheen, tape-test adhesion of 4B or better after seven days, and film thickness within the supplier's range. Test one edge and one dark-streak area. For the rejected top, strip to bare wood; do not sand and recoat, because contamination may remain in the pores. Document the solvent wash and sealer batch for traceability.