Finishing in Hot Weather -- Hot Weather and Fishing: Yes! - Hot Weather and Finishing: Yuck!
by John Buries
All coatings are designed to dry at certain rates for specific reasons. Stains must be wiped off consistently, sealers must soak into wood and allow trapped air to escape, topcoats must have a smooth glassed out look.
On a hot, dry day or night, coatings dry too fast. Hot ambient air (mid-80+ degrees) causes the solvents in your coatings to evaporate faster than normal. Stains won't wipe properly and clear coats dry on the surface, seal off air bubbles from escaping, and don't allow coatings to flow out from the wet edge.
Hot, humid air is even worse--the air is filled with moisture, not allowing space for the evaporating solvents. This causes the coatings to dry slower, yet still skin over quickly. In addition, lacquers will absorb the moisture in the air, causing the nitro cellulose to turn white (blushing).
In most cases, adding a retarder solvent can correct these problems. Adding retarder or a slowly-evaporating solvent will cause the coating to dry at its normal rate. This will give you back that wet edge and keep the quality look you want, and also help to prevent blushing in lacquers. If a film has blushed, apply another coat of adjusted lacquer. It will re-dissolve the blushed white film back to clear.
Always keep a stock of various retarders on hand. They are easy to use and great for quick adjustments.
Remember: Different types of coatings require different types of retarders. Check the literature or ask your sales representative for proper instructions.
Remember: Adding retarders to coatings will keep films a little softer than normal. Let things dry an hour or two longer before re-coating, sanding, or stacking. Always test a few pieces before going to the next step.
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