Controlling Shellac Blush
An extended discussion of methods for curing blushing in shellac, and their various possible pitfalls. November 8, 2007
Question
I never really had a problem spraying shellac (both the dewaxed seal coat and the clear from Zinsser), but with the humidity going through the roof, I am experiencing blushing problems. I was thinking of either purchasing a lacquer retarder or the no-blush line from Mohawk. What is the best route to prevent this problem going forward? What is the best way to fix the blushed areas already sprayed?
Forum Responses
(Finishing Forum)
From contributor S:
Use propanol or butanol - propyl alcohol or butyl alcohol, AKA n propyl or n-butanol. Butanal gives more open time than the propyl. Shellac blush can injure the properties of shellac - try to avoid blush in the first place. Use no more than you have to in order to correct the problem. Start with 2% and work up from there. It can be used straight in a cup gun by misting a light coat on the affected surfaces and letting set for five or ten minutes. Apply as many times as necessary, but never flood the surface with either of them.
From contributor D:
Are you sure that it is blushing you are experiencing? Reason I ask is that if by chance you used any stearated paper between coats, the stearates could have been left/trapped behind and are showing up as a blushing appearance upon recoating.
From contributor C:
I have spent most of my finishing career in South Florida, and yes, the high humidity can and does blush shellac down there - take my word for it. Especially during the rainy season when humidity can reach 90% and more.
From the original questioner:
I am in the Midwest and the relative humidity has been above 80% lately. Should I use something like Behlens lacquer retarder to prevent this?
From contributor S:
Yes, you can - but that does not mean you should! Practically all lacquer retarders are single or composite glycol eithers, such as butyl cellusolve, with possible add mixes of isomerized types. Of course these will get rid of your blushing problems, but can be very problematic in their own. And can even (in some instances) cause wrinkling and lifting of the shellac! If used, you must let the finish set till it is hard cured - which could take days or even longer. And shellac should never be applied more than 2 or 3 mils wet, and no more than one coat as an undercoat. Shellac is not meant to build the finish, but to act as a barrier and sealer coat only! So be my guest and use your lacquer retarder, but you still would be much better off with the alcohols I listed before. If I were you I would do some serious research into the use of spray shellac as a first coater use. In Florida I seldom used shellac for this purpose and if I did, it was in the winter when relative humidity was low.
From contributor W:
I am in the Tampa Bay area and almost never use shellac. It always seems to be a headache. I use Mohawk products and keep a 5 gallon bucket of butyl cellosolve around as a retarder. Ditto to all precautions mentioned. The hard dry time increases dramatically. It takes at least 12 hours for two sided parts to dry well enough to flip and not leave marks later. We are able to spray in just about any condition of weather with proper care and procedures.
From contributor P:
Have you tried water based shellac? I've been using the Target shellac for about 8 months now and never have blushing problems. I still use either Zinnser's seal coat or dry flakes mixed in alcohol on furniture and do occasionally have problems with the humidity with that. But on cabinets, I spray the Target with no trouble whatsoever.
From contributor C:
Contributor P, do yourself a favor - contact Zinnser or Mantrose Hauser and talk to them about waterbased shellac. You'll find it is not a product you want to use. The reason neither company makes a waterbase shellac is because the process to do so hurts the good properties of the shellac resin, making it inferior to shellac cut in alcohol or traditional shellac films. Does it work? Well, test a panel of each with water and you'll see the inferior results pretty quickly.
From contributor P:
It's worked fine for me. I use it on bare plywood, such as the interior of boxes and shelving, then spray Target poly over the top. I've also been using it for when I use a linseed based oil stain between the stain and the topcoats - again, no problems. I've also been using the Garnet water based over dye stain to deepen the color.
It doesn't give wood the warmth or the color that alcohol and shellac does, though, and it raises the grain considerably. But I think it does a good job of sealing and provides a good base for the CV and poly from Target that I use.
Don't get me wrong, I love Zinnser's and use it all the time. But the water-based is safer and cleaner to spray, and because I prefer the Target clears, I think it's better to stick with their product. Plus I've heard, but not experienced, that the Zinnser's doesn't work so hot with some of the water-based clears. I believe it's said there's some sort of pH problem.
From contributor R:
Like the original poster, our humidity blossomed. When blush first appears, heat it gently with a heat gun. The blush disappears immediately, and doesn't seem to return. Not standard operating procedure, but beats starting over. As an experiment, I took a piece of scrap and checked the surface temperature with a laser thermometer - 75 degrees. I brush on a coat of shellac (Zinnser dewax) and the surface temperature immediately dropped ten degrees with signs of blushing. Another poster suggested misting with alcohol to slow the drying process. After misting, the temperature dropped another ten degrees and the blush exploded. The heat gun cured it. The label says not to apply if temperature is less than ten degrees above dew point for good reason, and I was pushing it. Even heating the sample board in the sun first didn't help. Needless to say, I cut a hole in the wall, installed an air conditioner, and found a different way to seal the wood.
From contributor C:
The alcohols I suggested are slow evaporating alcohols, not ethyl alcohol. They act as retarders and what is used in the conservation field for shellac touchup and other things when more open time is necessary. Yes, grain or ethyl alcohol and especially denatured alcohol from local stores will decrease surface temp, and since they are hydroscopic, absorb moisture from the humid air and deposit it on the surface as it evaporates. But propyl and butyl alcohols act much like the glycol eithers and lacquer retarders - this is why they are used instead of the slower drying glycol eithers which keep the coating softer for much longer. As to heat, it can be very affective, but one must take care not to bubble the finish with too much heat! This will not ever be the problem with my recommendation.
From contributor R:
The experimenting I had done was to understand the nature of shellac. Another experiment I tried was to heat the sample in the sun (120 degrees top surface, 104 degrees underside). The sample was then sprayed. As predicted, the surface temperature dropped ten degrees but the blush returned even though the sample surface was well above dew point temperature. This suggested the problem took place between the gun and the surface. The heat accomplished two purposes. Evaporating the moisture and flashing the surface preventing additional blush. I'm not advocating the use or non-use of shellac, but if using shellac is important, the previous suggestions may prove helpful.
From the original questioner:
Thanks all for the discussion. I just tried the heat method and it was amazing to watch the blush vanish.
