Troubleshooting Solvent Pop

Even with state-of-the-art equipment, a finisher gets those pesky micro-bubbles in conversion varnish finishes. Here, pros put their heads together and try to figure out what's happening. December 30, 2005

I have recently started spraying M.L. Campbell's Krystal conversion varnish. I have also recently started spraying with my new Kremlin 20-25 pump with the mvx gun so I have to get used to both items at the same time. The biggest problem that I am having with the Krystal is what I think is solvent pop. I am getting these very tiny bumps in the finish that won't dry out.

I am spraying in a new paint booth with an air make-up system. I have a new air compressor with a refrigerated air dryer and pre and post inline water filters to remove the water in the air. I am using a 09-094 tip on my mvx gun. The temperature in the spray room has been in the 70's. I am first spraying on a coat of M.L. Campbell's water white vinyl sealer, then sanding it with 220 grit sandpaper, then spraying on a coat of Krystal.

The Krystal has first been catalyzed and then had 20% flow enhancer #2 added. This was the maximum M.L. Campbell rep said I could add. I have sprayed light coats, heavy coats, and taken the freshly sprayed part out of the air flow in my paint booth once sprayed to prevent it from skimming over too soon, and have held the gun approximately 6" from the part being sprayed and nothing seems to consistently solve the problem.

These are not always big bumps (sometimes they are), but you can feel them when you run your hand across the parts (feels like fine sand particles). On my Kremlin pump, my fluid is set at 40 and my air is set at 15 (recommended by M.L. Campbell rep). I don't know anything else to try. I have invested all this into this paint booth and equipment and would like to get great results.

Also, I want to use the booth and air make-up system, but I've had some technical people tell me that the air flow could be causing the finish to skim over too fast. So, what's the purpose in the booth and air make-up system? I've been spraying mostly lacquers with conventional and hvlp guns for the last 14 years and wanted to start using varnish and Kremlin pumps and guns, but just want to figure this problem out. Any suggestions would be greatly appreciated.

Forum Responses
(Finishing Forum)
From contributor R:
I believe the 20-25 pump has a 20:1 compression ratio. With 40 psi of fluid pressure that would give you 800 psi at the tip. Normally that would be way too high for Krystal thinned 20%. Ideally you want to be in the 400 to 550 psi range at the tip. With a 20:1 pump that would be about 20 to 28 psi fluid pressure. Too high of a fluid pressure could lead to micro-bubbles.

I think your tip might be a little small also. Try going up to the next tip size. Too small of a tip could lead to dry spray. AAA pumps are always about balancing fluid pressure against tip size. I think you will find that with a larger tip and the reduced pressure you will be able to move a little faster and still lay down a nice wet coat. At 70 degrees and the proper size tip, you should be able to use the standard lacquer thinner, 10 to 15% reduction.

From contributor B:

First I don't think it's necessary to thin 20% with a Kremlin setup. Krystal is not a heavily-solid liquid. Second, you need to sand with 320 on the first application, clean off, sand, dust and apply a second coat for the smoothest layer.

From contributor W:
Several thoughts come to mind here. The first is that 800 lbs. of fluid pressure is high for 20% reduced conversion varnish; I'm wondering if your Campbell rep wasn't thinking of the more common 10-14 pump with its 10:1 air/fluid pressure ratio rather than your 20-25 pump with its 20:1 ratio, which would mean that you should run your 20-25 pump at 20 psi. We usually run 10 psi. kinetic atomizing air, but contact your Kremlin rep for a tutorial of how to set the atomizing air for the most efficient pressure.

Second, is your material strained before you apply it, and are the filters the right size mesh for your application? The 20-25 has four possible screens: the filter at the end of the intake hose, the big filter at the pump (adjustable size mesh), the in-line filter at the gun handle (also adjustable size), and finally the optional screen at the tip. Remember that conversion varnish, once set up, doesn't re-dissolve, and it's possible to have little grains of hardened (or in your case semi-hardened) finish floating around in your bucket. Also, since your pump is new, have you flushed it thoroughly? There might be some contaminant in there.

Third, what substrate are you using? Solvent pop usually happens with large-pored woods like mahogany and oak, or with very thin veneers that allow a chemical reaction between the finish and the veneer adhesive. It usually looks like small bubbles rather than bumps or, if the bubbles pop, they leave small rings.

From the original questioner:
I am going to lower my fluid pressure, and try the next larger tip to see what happens. I think that my Campbell rep might have thought that I had the 10-14 pump instead of my 20-25. I know that Kremlin makes a good spray gun and pump and Krystal is supposed to be a good varnish.

I figured that it was a setting and/or tip size problem. I did spray some Krystal today with my old settings and it seemed to do great. The only difference that I can tell from when I sprayed and had problems and now is that the temperature is about ten degrees warmer. Before the temperature was about 73 degrees and now it is about 83 degrees. Could that have made any significant difference? I also have inline Kremlin heaters. When the temperature drops, would maybe setting the thermostat on the heater to 83 degrees or so help?

From the original questioner:
Contributor W - I forgot to mention the substrate that I am spraying on. I am spraying the Krystal on maple that has been stained with M.L. Campbell's microton dye and M.L. Campbell's woodsong II wiping stain.

From contributor S:
I am waiting to hear from others with air make-up units about how they deal with the finish-skimming issue. Having three air exchanges a minute is fine for OSHA and other regulatory agencies, but it’s tough for a finish that has just been sprayed and all it wants to do is to be left alone with no turbulence so that it can flash off.

From contributor M:
To the original questioner: What is the sheen of the Krystal? Dull?

From the original questioner:
The sheen of the Krystal that I have been spraying is satin. For you guys that use air make up systems, how do you solve the problem with the finish skimming over too fast because of the air flow? Do you just turn the air make up system off after you spray until the finish flashes off and then turn the air make up and exhaust system back on?

From contributor M:
To the original questioner: How did everything turn out with your resistant? The reason why I ask about the sheen is that we are experiencing the same problem with some Krystal that we've been trying to spray recently. We however have been spraying a tinted dull. Our supplier is telling us that it might be caused by too many solids in the product due to the dull sheen. We've tried everything, and still are having problems.

Does your finish look like tiny pebbles? Very, very small, not particulate in the finish, but really something wrong with the finish itself? We have tried everything, all of the gun settings, tip sizes, booth air flow, heat, etc, etc and nothing works. It has made us very frustrated because there has never been a problem like this that we could not fix. We've tried thinning, retarding, reducing and nothing happened. We’ve also tried spraying light, spraying very heavy and still nothing. We sprayed a whole kitchen with this finish and everything was fine, then when we got a few more gallons when the customer added a few items this problem started. We ended up fixing this by sanding and then spraying a coat of Duravar dull over the top and everything came out fine. However we have never figured out the problem. I'm thinking, because we had sprayed so much successfully that it could be a bad batch that M.L. Campbell’s has put out, although I'm not sure.

From the original questioner:
The resistant ended up turning out pretty nice. There were these "micro bubbles" in places, but it was acceptable. I seem to have the same problem with the Krystal and the resistant. I really want no "micro bubbles" in my finish. I want it to feel as slick as silk off of the gun. Sometimes I get that and sometimes I don't. These "micro bubbles" are very frustrating to me too. I have been spraying finishes for the last 14 years and I am so ready to get this finishing down to a science and not just a flip of the coin. I have sprayed some very nice finishes, but I get frustrated when the finish doesn't come out the way I think it should. Like I said in a previous post, I have invested a lot of time and money into my new paint room and equipment. I did that to try to rid some of these problems.

The "micro bubbles" that I am getting in my finish are like extremely tiny bumps. They are like little specs in the finish. My Kremlin rep came by my shop today and I talked to him about this problem. He was wondering if the "micro bubbles" were specs of dust, but I told him that I am sure that is not what it is. I am sure that they are either extremely tiny air bubbles or solvent pop. After you've sprayed as much finish as I have you get a feeling for these things and just know what to watch for. I am hoping that by turning down my fluid pressure on my Kremlin pump that the problem will go away. I guess only time will tell. I asked my Kremlin rep about going to the next larger tip size and he didn't think that would help. He said I would be putting more fluid down and that I would probably still have the same problem. But, I may get the larger tip and try and see what it does. It might be a good idea for you to call M.L. Campbell's lab department and ask them to look up your batch number on your finish and see if they can tell if there is a problem with that batch. Finishing can be fun when it goes right, but it can be one of the most frustrating things when it doesn't go right.

From contributor R:
Finishing is one big multiplication problem. The variables include: moisture content of the wood, sanding schedule, gun settings, tip and air cap size, atomizing air and fluid pressure, compressed air quality, temperature and humidity, viscosity, finishes and solvents, hand speed and target distance to name a few.

Finishing is a system. Every physical, chemical, procedural and environmental element in the system has an effect on the final outcome. Change some variables a little and you can create major havoc. Some you can change a lot and see no major effect. Some change and we unconsciously compensate by changing something else (gun speed and target distance for instance).

It can get frustrating when we think that we are doing everything the same as we always have and there are problems. This is the point that we have to start looking at the other elements in the system, perhaps environmental or procedural that has changed. Did the temperature change or did someone different spray the job, that kind of stuff. Sometimes the changes are obvious, sometimes they are not.

I find that I am most successful at analyzing a problem when I "think like paint". Stated another way, if I were paint what would make me do that? For instance micro-bubbles, what are they? They are tiny bubbles, typically air or solvent that become trapped in a finish. Ask yourself how did they get there and why are they trapped. Too thick of a film, too fast of a surface dry, too much atomizing air, wrong solvent for the temperature, etc. Now go back to each of these individual reasons and look at what causes them. Too thick of a film could be affected by: tip size, fluid pressure, hand speed/ target distance and so on.

Finally, compare your "system" to the each of reasons for failure. Experience will quickly eliminate many of them. It gets tough when there are combinations or multiple combinations of reasons within your system, such as tip/pressure and temperature/solvent. Take a slow, logical and methodical approach and you will find solutions.

From contributor G:
I had the same equipment and the same problem w/SW. Their tech guy came out and said the problem was air entrapment (the bubbles) in the finish. He added a small amount (1oz/gal) of what he called a "bubble buster" into the finish and it worked great.

From the original questioner:
To contributor R: You are right. It can be very frustrating trying to figure out what causes the different problems in finishing. If I maintain consistency on my part of the finishing, then there has to be some other variable causing the problem, whether it is temperature changes or something else.

From contributor J:
To the original questioner: Were you able to solve this problem? I was scanning through the forum and saw this. The 20-25 pump is better than 10-14, it’s just that you were using opposite combination from M.L. Campbells's sales rep advice regarding fluid pressure and air pressure. You should be using 35-40 psi air pressure and 15-30 psi fluid pressure. You will have a good atomization with this combination.

The key to proper spray application is atomization. If I am not mistaken you had that problem because the paint is not distributed evenly as you spray and 40 psi fluid pressure is too much which is enough reason for your lacquer to pop. I have used a lot of Krystal lacquer and I never had any problems even in porous wood.

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

Comment from contributor A:
If the bumps you feel are raised it could be sanding dust. Vinyl dust likes to stick to itself. After sanding with 320 I use a maroon Scotch-Brite pad and smooth it out again. Then before shooting I blow it off, wipe with a rag, blow it, wipe with a tack cloth and blow once more. If it is truly micro bubbles then the finish is too thick. The settings I use on my Kremlin are 12 lbs air and 30 lbs fluid.

Comment from contributor R:
What you are experiencing is solvent pop - trapped solvent due to over application. Use thinner coats with a proper flash time between coats. When you apply coats with the wrong viscosity or too fast hardener or solvent, the top surface of the film closes trapping solvent that eventually comes out through the top surface of the film as a blister or pop.

Comment from contributor O:
Try spraying the Krystal with no C10025 water white vinyl sealer. Sometimes you can catch a recoat window, so try self-sealing, or try the Krystal Sealer. I have had some of the same issue recently. The gun I was using was the new Graco G15 20/1 pump - fluid at 35/Air at 12psi. Try lowering your air pressure. Your finish may start to kick over too quickly. Do you get any kind of white dry spray on the two horns of the gun? That would be the product drying to fast and too much air at the tip. You always start your fluid pressure around the same and then start with your air pressure at around 10. Get some cardboard, and keep hitting the trigger and upping the air pressure until your fan is evenly displaced.

Comment from contributor F:
Being in the automotive paint industry for a few years, I have experienced more than my fair share of solvent pop issues. One major factor that affects solvent pop that we tend to forget about is low humidity. If your booth humidity is less than 50 to even 60% "some" resins have the tendency to skin over too fast and result in solvent pop issues. If you have a way to control humidity in your booth bring the level up. If you don't have the ability to control humidity bring your temperature down. This will increase your cure time which will avoid skinning over too quickly. If you don't have control over heat or humidity, get some. The only other way to control solvent pop is with different solvent packages which your paint supplier should be able to supply to you. If not, you need to find a supplier who can.