When you apply lacquer, shellac or another quick-drying finish in humid conditions, the finish can turn milky because the moisture trapped in the finish didn't have a chance to evaporate before the finish hardened. Polyurethane doesn't exhibit the same behavior -- not even the water-based type. When the finish is hazy, it's usually a buildup of zinc oxide, which is used as a flattening agent. There's no easy way to fix it. This is true for oak, pine or any other type of wood.
It's Important to Stir
Both water- and oil-based polyurethane are varnishes, and they dry to a naturally glossy finish. To create a satin or matte finish, manufacturers add a flattening agent -- usually zinc oxide -- which disperses reflected light and cuts the gloss out of the sheen. This material collects at the bottom of the can, which is why it's important to stir well before using it.
When the finish is hazy, you made one of two mistakes:
- You applied too many coats of satin finish.
- You applied material from the bottom of the can rich in flattening agent that had not been properly mixed into the varnish.
What to Do
Before getting upset, check whether the polyurethane has actually dried. If you're using water-based material, it's common for it to appear milky before it dries. The color comes from the emulsion that carries the resins. If the finish is still wet, just wait -- the milkiness should disappear.
If the finish has been drying for several hours and feels hard to the touch, though, you've got a problem because there is no easy way to correct a buildup of flattening agent. Applying another coat of finish won't remove the cloudiness as it does when you're applying lacquer because, unlike lacquer, polyurethane cures. Once it's dry, it won't re-emulsify. Adding more finish will just make the cloudiness worse.
Whether the cloudiness is caused by flattening agent or moisture, if you can't live with it, you'll probably have to strip, scrape or sand the polyurethane varnish and do it over. If the cloudiness isn't widespread, and it's caused by moisture, a simple trick may help:
Cover the affected area with mayonnaise or petroleum jelly.
Wait overnight, then wash the wood clean. You may notice an improvement because the oil in the mayonnaise or petroleum jelly leaches moisture out of the finish.
Repeat if you noticed a difference but the cloudiness isn't completely gone.
Preventing a Recurrence
A few tips can help you avoid having this problem happen in the first place:
- Stir the material you're using until the tip of the stir stick is no longer covered with white residue when you scrape it on the bottom of the can.
- Allow wood to dry completely before applying polyurethane. Test it with a few drops of water -- if they don't immediately soak in, the wood is too wet to paint.
- Avoid building up coats of satin or semigloss finish. If you need to build up coats, do it with gloss material, then use satin for the last one or two coats to cut the sheen.