Q: I sprayed a very thin final coat of lacquer on a guitar when it was cold and humid, and the finish blushed. I had to spray another coat of thicker lacquer and the blushing went away. I think I understand the reason why blushing forms, air trapped, but what I don’t understand is why thinner lacquer makes it worse.
A: Blushing is not caused by trapped air; it is caused by trapped water. When the relative humidity is high, no matter whether it is hot or cold, you can get blushing. Here’s how it happens.
High relative humidity means the air is holding almost all the moisture it can hold. As air gets colder it is less able to hold moisture. Thus, if the temperature were to drop just a few degrees, you might just hit the dew point; that point at which the air is overloaded, relative to its temperature, and moisture starts to come out of the air as condensation.
When you spray a very thin coat of lacquer, the abundance of thinner flashes off very quickly. This process of evaporation uses energy in the form of heat. Thus, it robs heat of the surface and the immediate surrounding air, making the air just above the evaporating solvent a bit cooler. Sometimes it is just enough to condense the water in that air, and that condensation gets trapped in the lacquer as blush.
It should be obvious now that the way to avoid blush, other than not spraying when the air is laden with moisture vapor, is to slow down the evaporation rate of the lacquer, and in fact, one of the most common blush eliminators is simply slower evaporating solvent. That way you can still spray as thin a coat as you like, but without the blush. Does that help?
Michael Dresdner
straight talk about wood finishing
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