Preventing Blushing of Pre-Cat in Humid Weather

      White haze (blushing) in pre-catalyzed finishes is a common problem in hot, humid weather. Here are tips on preventing it, and correcting it when it does occur. December 9, 2010

I am spraying Sherwin Williams pre-cat (T77) dull rub over their wiping stain on some pine trim and birch doors. I am fighting the humidity and heat in my shop and lately I'm noticing a finish turning white and cloudy in areas. Looks terrible. I am laying it down with an HVLP about 4 mils wet per coat. Thinning it about 10%. Any ideas what can cause this?

Forum Responses
(Finishing Forum)
From contributor R:
Moisture in the air causes the white haze you're seeing. Some call it blushing, and you can eliminate it by mixing in some retarder into your top coating. Another thing that can cause a whitish look once you have sprayed on a coating, is that your stain hasn't dried enough. When you spray on the coating too soon, the solvents in your stain start migrating through the coating. Try a bit of retarder - I've got a hunch that will cure your blushing issue.

From contributor L:
Since it is a pre-cat, you can always spray retarder on the piece to redissolve the lacquer, which will remove the blushing. This will only work if you do it within a few hours of spraying. Make sure you let it dry completely - do not touch it before it dries. Don't ask why I know this.

From contributor R:
Contributor L, I bet you only made that mistake once.

Years ago some Harvard/MIT scientist type person spent a boat load of taxpayer money to prove that a "wet paint" sign is the most touched and most ignored sign.

A boss of mine came in the spray booth one afternoon after I had just coated a door and I was behind the workbench cleaning the spraygun. For reasons unknown, he touched the damn door, leaving a fingerprint... plain as day. He didn't see me since I was at the other end of the shop, but he looked all around, put his hands in his pockets, and then left the spray room. I asked the next day if anyone had touched the door and no one 'fessed up. I got a chuckle out of that and kept what I had seen under my hat.

From the original questioner:
I did mine on an older dresser that someone had put a hot water humidifier near and blushed a large area of shellac on the unit. I sprayed one of the drawer fronts with DNA and it removed the blush almost immediately. I let it sit for a while and gave it a touch test when it looked dry. It wasn't and I pushed the shellac and caused a large wrinkle. So I took a rag and wiped all the shellac off the piece and resprayed it. Lesson learned.

But the retarder should work to remove the blush. Or you could buy the blush remover at four times the price, which will do the same thing and is just retarder anyway.

From contributor J:
Well, I wasn't going to say anything out of pride, but... yeah, a huge palm print dead center of one of the doors. The cops couldn't have done a better job.

If retarder is likely my solution, what do you think about just shooting another coat on with no air moving across it? I usually put a fan on a few minutes after the pre-cat goes on and never had a problem until lately with the heat and humidity. At 4 mils wet with only 27% solids, I should still be under the DMT specs.

From contributor L:
If you don't remove the blush before you put another coat on, you will trap it there. After the coating has flashed and it is touchable (usually 5-15 minutes) you can put a fan on it. If you do it before it flashes, you run the risk of getting solvent pop.

From contributor P:
I agree you should put in a little retarder. I would use up to 5% butyl cel. More and you can get into more issues.

It might also be water in your air line. When things get hot and humid, it's pretty easy to get a lot of condensate in the air that we use to spray. This can equate to blushing issues as well. You might want to ensure that your air/water separator is empty and cracked open a little bit. You can also make a home made air dryer by looping the hose against a wall. At the bottom of each loop, put a drain cock. Open the drains once a day and drain the water off. The loops should look like the back of a fridge. If neither of those work, you might want to look at Lorchem's Unilac. It's pretty blush resistant and works in extremely humid areas like south Florida.

From contributor Y:
That is a pretty common problem around my shop. When I don't have retarder to add to the top coat, I'll spray with the booth exhaust off. I find that in hot and cold weather, direct air from a fan or draft will cause the blushing.

From the original questioner:
I found that to be exactly true. I experimented with the trim a bit and even in 90 degree heat the blushing stopped altogether after I stopped the fan while the lacquer flashed.

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