A member asked me a question today about spraying polyurethane, and it reminded me of something that I think you should know about. A while back another member came to me with a catastrophic finish failure on a walnut coffee table. The finish, which happened to be Varathane polyurethane in a can, turned milky white shortly after applying, and after two weeks it was still white. The finish had been applied in a garage, and both the finish and the table in been in there for a couple of days prior to spraying. The only remedy at that point was to totally strip the piece and start over.
So what happened?
The short version is that the finish had “blushed”, which is a condition caused by moisture being trapped in the finish and unable to escape. Sometimes it will correct itself in a couple of days, but that was not the case here.
In our humid environment this is a real concern, and something to be taken seriously. This can happen with almost any type of sprayed finish, and it doesn’t matter if the finish comes out of a can or via a HVLP spray rig, although my theory is that finish in a can is more prone to it. And here’s why… When you use an aerosol can we all know that there’s a propellant in there, and when that propellant expands and vaporizes (as it must) it gets cold. That’s enough to condense moisture in the spray stream, and it goes directly onto your workpiece and into your finish.
With a HVLP spray rig you can still capture enough moisture to be a problem, so that’s not the answer either. The best bet if you want to spray is to pick a day when the humidity is low, or better yet do your spraying in a controlled environment where the humidity is low.
With some finishes, like lacquer, you can use a retarder to slow the drying process and allow the moisture to escape. To me that seems like a lot of effort and complication, so I’d just try to pick a dry day.
If you’re curious what blushing looks like I’ve attached a picture that I snagged somewhere on the web.