Spraying shellac

Removing water, which damages appearance of shellac, from air compressor output. May 2, 2001

Question
I've been having some trouble spaying shellac. When it gets humid, I get water damage in the shellac finish. I have a filter on my air compressor, but sometimes water gets through it. I have a belt drive 5.5hp two-stage compressor.

What is the best way to remove the water from my air compressor output?

Forum Responses
The moisture extractor should be at least 25' away from the compressor. You need some distance in order to allow the moisture in the air to condense. After the extractor, you connect the hose to your spray equipment. I don't think an extractor does very much good located on or close to the compressor.

Also, it helps to incorporate a simple drop leg of pipe with a drain valve to collect moisture in your air line before it goes through your extractor. This way your extractor won't have to work as hard. Again, put this downline at least 25' from the compressor.

If you still have a little moisture getting through your extractor, you can get a disposable in-line air filter. This is a small filter that installs at the end of your hose that connects to the gun. Ones I use are little blue plastic-cased things made by Sharpe. They come 2 to a package.

I'm also told if you use a better grade of alcohol when you mix shellac, it sprays better and helps prevent the blushing.


That's right on target about the drain leg, however the reason you want the extractor more than 25' away from the compressor is heat. The air temperature leaving a recip is about 270 degrees and you need that run of pipe to cool the air. In fact, to rid yourself of most of the moisture, a refrigerated drier is the way to go. It chills the air to about 0, then reheats it. The cooler temperature lets the moisture drop out of the air stream. These cost but are worth it, depending on your needs.

The problem is probably not water in the lines, though you should heed the advice of the above posts. The problem is also in the humid air. The rapid evaporation of the ethanol in the shellac quickly chills the air directly above the drying finish and this condensation kicks the shellac resin out of solution, causing the white appearance (blush). The same also happens with lacquer.

A lacquer retarder added to your shellac solution will help, as will using a slower evaporating alcohol like isopropanol, which can be added to your existing finish. Also, spray lighter coats.



From the original questioner:
The moisture is condensing in my conversion gun (a Lexaire 2002). I'm going to try Air Max Procuts' coalescing filter and dryer. I don't need a regulator, as my 2002 has one built in.

The above is exactly right if you are experiencing blush (finish turning white after you spray it). Blush does not occur because of moisture in your air lines. It happens because of the room air conditions. Moisture in your air lines will cause bubbles in your finish, not blush.

Just make sure the drier portion of the system is sized for your compressor output in cfm. Remember, don't oversize it because the velocity will down in the unit and reduce its efficiency.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor J:
A way to avoid blushing in bleached shellac ethanol solutions is to add them ammonia in order to decrease surface tension on the film. Solutions must be diluted to 10% of solids or less to avoid "orange skin" microgelation trouble. Decolorized shellac solutions can be treated with ammonia regardless of concentration probably because natural polymer is less acid (pH at 10%w/w 4.77).

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