I am having problems with my finish. I am using a nitrocellulose gloss lacquer. It has been raining today and it’s about 60 degrees in the shop. I keep getting a milky white translucent discoloration with an oil spot quality. I am spraying over about 3 coats of sanding sealer. It appears that a very fine layer of condensation is collecting on all of the recently sprayed finish. Is this the cause of the milky white spots? This has never happened before to me and I have been spraying for years.
From Paul Snyder, forum technical advisor:
Going years without ever seeing blush is incredible good luck. To fix the discoloration, you can fog on a very light application of lacquer retarder or spray another coat of lacquer that has some retarder in it. A general rule for spraying lacquer (or shellac) is don't spray if the relative humidity is above 65%. A more accurate guide is don't spray if the dew point is within 20 degrees of the ambient temperature. For example, if the dew point is 65 degrees and it's 80 degrees in your spray area, don't spray unless you use retarder. You need more than a twenty degree difference between the dew point and temperature to be safe.
As the solvents in lacquer evaporate, they have a cooling effect on the surface they're sprayed on (put a few drops of isopropyl alcohol on the back of your hand to feel the effect). The cooling effect can be as much as twenty degrees. If you cool the surface down below the dew point, water will condense on the surface and you'll get blushing – the milky white look in the lacquer/shellac.
Also make sure the piece you're spraying is the same temperature as the environment where you're spraying. I once made the mistake of taking a piece from an air conditioned space and spraying it in the outdoor heat. The piece was a lot cooler than the outdoor temperature and the finish blushed almost instantly. In order to spray lacquer in high humidity, you'll need to add lacquer retarder. Lacquer retarder is a slowly evaporating solvent (thinner) that lets the moisture escape from the lacquer before it dries. Only use a small amount of lacquer retarder in place of the regular lacquer thinner. Use the same brand of retarder as the lacquer you're spraying and only use as much as the manufacturer recommends. For example, if you usually thin the lacquer 15% with lacquer thinner, and the manufacturer recommends 5% retarder, switch to 10% lacquer thinner and 5% lacquer retarder (15% total thinning as usual). Adding lacquer retarder causes the lacquer to dry more slowly than usual. Spray your coats a little lighter to avoid runs and sags and allow more time between coats.