Micro-Bubble Solvent Pop Fix (Again)

Okay, one more time: Spray thinner coats; use a retarder. June 28, 2006

Question
I am having trouble with micro bubbles in my clearcoat. I self seal with a conversion varnish. I spray with an A.A system Graco pump and Kremlin mx gun. Any advice would be greatly appreciated.

Forum Responses
(Finishing Forum)
From contributor M:
Here’s a list to use for troubleshooting:
1. May be water in your air supply system.
2. Maybe you are putting on a more than needed thinner or catalyzer.
3. A top coat that doesn’t mach the base coat?
4. The base coat is not dry enough.

Try to eliminate these things. Have you had this problem from the beginning? Does it happen when you change material?


From the original questioner:

No I’ve been using the same conversion varnish for a couple years. I tried 1 oz 2butoxyethanol per gallon and that helped a bit. I think I’m going to try to reduce it with thinner.

From contributor GD:
What size tip are you using? What are your pressures? Try going down in tip size and cheating a little by turning air up a few notches

From contributor P:
A very simple solution worked for me: hold the gun at least 10 to 12 inches above the surface. I have a Kremlin 10-14 with MX and spray mostly Durovar or Fuhr 255. I found that I was getting pinholes, micro bubbles, micro-orange peel and etc. even after correct viscosity and getting rid of the "tails".

From contributor D:
I added Flow Coat from Restoreco Kwick Kleen and it stopped the micro bubbles. I have an AA Kremlin System also. It keeps the top of the surface open longer to release the air trapped under surface.

From contributor E:
There has probably enough been said already but I can't resist pointing out that the last 2 posts contain the essence of what it takes to get rid of micro bubbles in 95% of the cases where it occurs.

1. Spray thinner coats.
2. Add a retarder.

This subject comes up a lot and the archives contain a lot of good information (search "microbubbles" and "solvent pop") but they also contain a lot of miss-information and speculation. (Differential pump stroke displacements will not inject air into a coating.) If you sort through it all to see what really worked, I think you'll find it's almost always a change that resulted in thinner coats and/or with the addition of a retarder.

The bubbles result from solvent that is trying to escape but gets locked in when the coating skins over. It seems reasonable to me that if there isn't as much solvent to evaporate and if it doesn't have as far to go to evaporate and if I can keep the coating from skinning over prematurely, I won't get bubbles. There are many factors that affect wet film thickness but one that is often ignored is viscosity: you must know what viscosity your setup sprays best and be able to produce it repeatedly. You should also be able to measure your wet film thickness.

If you think you've done everything right and still get bubbles, add a retarder. If you add a retarder and still get bubbles make a change that will reduce your wet film thickness (wider fan, more stand-off, smaller nozzle, less fluid pressure, move faster, etc). I like to stay below 4 mils wet but have seen it necessary to stay below 2 mils. It depends on the particular coating you are using. Obviously there are other things that can cause micro bubbles. But I'll wager that "piling it on" is the biggest culprit.


From contributor R:
Sometimes if you spray in a cold temperatures and move the piece into a forced hot air room, you can get bubbles, especially if you lay it on thick, which you tend to do with air assist. Next time it happens, put the same finish in a compressed air gun, spray it and keep it at same temperature and see if that’s the cause

From contributor J:
I have found that if you do a very fine mist coat and let that dry before you spray your primer coat, it allows the gasses to escape without getting caught up in the finish. Then, after a minute or two, when it flashes off you can spray like you normally would but without the micro bubbles.

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