A Case of Blushing in a Cherry Cabinet

In humid conditions, cooling as the finish dries pulls condensation into it. March 18, 2005

I'm having a problem with the finish on a recent piece. The area around each opening has, for lack of a better word, evaporated. It appears as if there is no finish there and a slight grayish tinge to the area. Is this blush? I have never encountered this before, but I will be the first to admit that I am no finisher. However, I seem to do a lot of it. For the record, the piece is oiled cherry, with sanding sealer and top coated with WW nitrocellulose lacquer (rubbed effect). This problem did not appear on the sealer coat, only on the top coat. The first coat developed this problem so I sanded it and shot it once more with the same results. It looked fine going on but as it dried, it manifested itself again. Also, for the record, it was a bit humid and cold that day, but other pieces I shot at the same time had no problem. I am baffled and turn to the Finishing Forum brain trust for guidance.

Forum Responses
(Finishing Forum)
From contributor J:
What you're experiencing is called blushing. This happens when moisture (water) is absorbed from either the atmosphere or objects placed on a finish. To remove this, you'll have to safely warm up your shop before finishing is done and try to maintain a steady temp. Since you have already topcoated, you can load a cup gun with lacquer thinner and spray, which will allow moisture to release out of finish. The use of a retarder also will aid in preventing blushing, but at the cost of slower dry times.

From contributor B:
You said "oiled cherry". Did you apply an oil on the wood before you did the sealer?

From Paul Snyder, forum technical advisor:
Good example of blushing.

When the solvents in the lacquer quickly evaporate, it cools the surface. Put a little alcohol on the back of your hand and let it evaporate; it'll feel cold. If the dew point is close to the ambient temperature, the cooling effect of the evaporating solvents causes condensation (dew) to form and it gets trapped in the finish. The moisture turns the finish milky.

I'd also use a little retarder to avoid the problem or wait until the humidity goes down, or turn up the heat in your shop and warm up the pieces you're spraying. With any of these fixes, the next coat should take care of the problem.

From the original questioner:
Thanks for the input. I will try the remedies. What would happen if the lacquer was heated prior to application? Would this help to avoid this in the future? My shop is difficult to heat - it just doesn't stay around very long. Also, why did it blush only around the openings and not over the whole piece?

Yes, oiled prior to sealer coat and cured for about a week.

From Paul Snyder, forum technical advisor:
Heating your lacquer can make it worse. That'll increase the temperature difference. It's the temperature of the piece you're spraying that counts. A radiant heating system that keeps solid objects warm is an option you may want to look into.

If heating isn't an option, then just use the minimum amount of retarder needed to cure the problem. It depends on the lacquer, but I can usually get by with 2% retarder and go up to 5% when the humidity is really high.

It's pretty normal to see blushing concentrated at edges and corners. Seems to be the way the air moves over a piece as it's drying and the evaporating solvents create their own air movement. I've had pieces blush just on an end where they were exposed to moving air (air flow into the booth).

From contributor R:
It may be caused by a different sanding grit. If the edges were sanded with a coarser grit paper or the flat surface was not sanded as much as the corners, this could contribute to a different evaporation rate. Also, the finish thickness is different on the corners than the flat area. The will also give it a different evaporation rate.

From contributor D:
We have had similar results with oils on wood. Cure was to dry the oil for up to two days. Oil that penetrates is not drying and causes the sealer not to bind. You could be seeing a delamination. Try scraping the area with even a fingernail. If it strips easily, your sealer has not bonded.

From contributor V:
The reason corners and edges would be more prone to blush is because they cool down faster than interior areas. On an edge, there is twice as much surface area in relation to the volume of wood, and thus a much greater cooling effect. Of course, air flow will also have a large effect.

The same principle applies to the charcoal in your grill, and the steak on your plate. The edges cool quickest.