Solvent Pop Versus Air Bubbles

Solvent pop and air bubbles look similar, but are caused by different mechanisms. In either case, the micro-bubbles are challenging to get rid of. January 27, 2007

We have been having occasional problems with hazy clouded areas when shooting clear coat CV on top of dark finishes. Looking closely, there are many very tiny bubbles in the finish that are not releasing before the finish sets. They are more common where the wet coat is thicker. We have not had these problems before changing to 275VOC. I'm wondering if this has something to do with the change in solvents? Has anyone figured out a way to deal with this problem?

Forum Responses
(Finishing Forum)
From contributor T:
Do a search of the Knowledge Base on "solvent pop." Heavy wet coats is one of the primary causes. Keep your wet coats less than 4 mils, add a retarder and keep the finished product out of an excessive draft until the finish sets. Temperature and humidity can also be culprits when they cause the finish to skim over prematurely (high temp and/or low humidity).

From contributor G:
I presume that in California, you have to keep to the low VOCs, and thinning or retarding would take you over the allowable max. What is the viscosity? Can you heat up the CV to make it thinner? What is the CV anyway? And how are you applying it? What kind of gun?

From the original questioner:
Just to clarify, these bubbles are so small they can not be distinguished without a magnifying lens. A mass of them makes the finish look cloudy. We were shooting Chemcraft CV, as well as Valspar CV and pre-cat. We are using a CA Technology AAA setup, 14-21 pump and desiccant air dryer. A couple of weeks ago we had a real problem with this - the weather was dry and warm. We tried thinning to 10% with lacquer thinner, xylene and butyl cellusolve.

From contributor T:
Lacquer thinner and Xylene in a high solids CV?

Here's the theory: The bubbles (if it's solvent pop) are cause by evaporating solvents that get trapped in the finish when it skims over before they can all get out. When it comes to skinning over, the finish doesn't know or care how thick the coat is or how much thinner is in it. It only reacts to the gradient between itself and the air over it. So if it's hot and dry and there's air moving over the surface, it will skin over sooner - often before the solvents have fully evaporated. It stands to reason that the more solvents that are below the surface, the longer it is going to take for them to evaporate. A thick wet coat will contain more solvents in the coat, which will take a longer time to evaporate, and thus carry a higher risk of solvent pop. Likewise, if you add thinner to the finish, you're adding more solvents that have to evaporate, which will take more time, and again you have more risk of solvent pop.

So: a retarder will help, but don't thin unless you absolutely have to, and then with the manufacturer's recommended reducer. Keep your wet coats thin - you've got a high solids finish that shouldn't take more than 2 coats, so you really don't gain anything by piling it on. And try to minimize air flowing over the piece until the finish has set. Some spray booths have a pretty good breeze going through them and it doesn't help a bit.

From contributor D:
It's almost certainly not solvent pop. I've had this happen with automotive clear coat and the reason I say that it's not solvent pop is that it happens instantly, whereas solvent pop doesn't. As soon as the gun passes the area, you can see the microbubbles in the freshly sprayed area behind it. To me, the problem is air entrapment and not solvent pop. I had a looser than I suppose it should have been nozzle on a gravity gun and it was inducing air into the paint.

From contributor M:
Contributor D is 100% correct. With AAA equipment, the only way I have eliminated it is with a Kremlin gun and the Ultra tips with the pre-orifice. I have spent years dealing with this issue!

From contributor T:
Certainly could be entrapped air. As contributor D suggests, if it is detectable immediately, it is more than likely air. Common cause is poor atomization, which is probably why the Ultra tips work for contributor M. Poor spraying technique (too close and/or too slow) can also contribute. Try cranking up your fluid air/pump pressure to see if you can get better atomization.

From contributor G:
Contact your Chemcraft and CA Tech reps. They should be dealing with this.

From contributor P:
I have a similar problem, micro bubbles, using ML Campbell Ultra Star water based finish, satin. Any reflections on this? Are the comments the same as with nitrocellulose lacquer? I use a Wagner capspray 5000 HVLP system.

From contributor J:
Wagner HVLP? Please. Contact a Kremlin rep for a 10-14 demo with the Ultra tip.