No matter what the season, both oil and waterborne polyurethanes need THREE conditions to cure quickly and thoroughly:
- low humidity
- air exchange
The air has to be DRY because heavy, moisture-laden air literally blocks oxygen from reaching the finish
It needs to be WARM because the chemical reaction that cross-links and cures the finish will not proceed below 65°.
And a constant but gentle source of FRESH, OUTSIDE AIR provides the oxygen that the finish must combine with in order to become a solid and fully cure .
Some tips for creating the perfect curing scenario:
If the interior temperature is likely to fall below 65 at any time during the coating or the first week after coating, turn on the heat!
Don't point a fan directly at the drying floor; it will force dust into the wet finish. Use a fan in an adjacent room like a bathroom or kitchen vent that is connected to the outside, and leave it on. In warm weather you can put a box fan in a window (blowing out!) in an adjacent room. In cold weather, box fans in windows will suck out too much heat - just gap the top a window in each affected room.
High humidity is the most difficult coating condition to deal with. Turning on the AC will bring the interior humidity down, but without opening windows, the evaporating solvents have no place to escape and fresh oxygen can't get in. But opening windows brings the humidity back up. Usually you simply have to resign yourself to longer times between coats and a longer overall curing time during the most humid months.
For oil-based polyurethanes, you should keep one window in the coated room gapped to 3” for two weeks.