Yet Another Look at Solvent Pop
Another cabinet finisher struggles with solvent pop. July 11, 2009
Question
I am looking for some really experienced finishers to help me with a finishing problem that I have been battling for a long time and would sure like to be able to figure this out and quit fretting over it.
I think that the biggest problem that I have had in finishing has been microbubbles (solvent pop). I have experienced this problem with M.L. Campbell's Krystal, M.L. Campbell's Resistant, M.L. Campbell's Clear Magnamax, and now with some PPG single stage paint that I am spraying. I’m spraying the PPG paint on a metal cabinet that I am refinishing.
I sprayed the wood finishes mostly with my Kremlin AAA sprayers. Now I’m spraying the PPG paint with a Binks 2001 gun and pressure pot. I know that most people say that you add retarder (Flow Enhancer #2 for the Campbell finishes) and it keeps the paint wet long enough for the bubbles to pop out. But, that really has never solved my problem.
When I was spraying M.L. Campbell's finish, I would go up to the maximum 20% of Flow Enhancer #2 and even up to 25% to see what it would do. Nothing would ever solve the problem. I had tech reps come out and we tried different pressure settings but nothing has really resolved this issue. Now I’m having the same problem with the PPG paint. I’m wondering if there is something that I don’t have set right on my gun, pot, and AAA system that is causing this. I’m at a total loss as to how to fix this problem. If any of you guys have experience with this problem and know how to resolve it, I would so really appreciate any help that you could give me.
Forum Responses
(Finishing Forum)
From contributor J:
Try spraying a really thick coat on a sample and see if it goes away. Do you have a wet film gauge?
From contributor A:
Here is some of my real world experience with your issue. Microbubbles and off-gassing could be two different problems but I will treat them both as the same for your instance. I have eliminated the problem by warming both the substrate bst (board surface temperature) and the coating to 85 degrees F. All you’re trying to do is lower the viscosity without adding solvents to help the bubbles to escape. Only heating one of either the substrate of coatings will not work. Higher heat usually only causes the coating to skin over.
From contributor S:
Contributor A has a point with trying to keep the substrate and your coating up in temp, is your Kremlin grounded? Are you spraying with #6 tip or smaller? What are your shop conditions like? What is the air flow through your booth like? To rule out the equipment take a small block of sealed wood and apply the top coat by hand. If you have a business card dip it half way into your material and swatch on to your block and watch as it sets up.
From contributor W:
I'm sure you know solvent pop occurs when your finish skims over before all of the solvent has evaporated. Excess air flow over your piece as it is drying can be a cause. We would get it when the booth filters were all replaced during dry winter months. Another cause is too much wet finish. The more you lay on the longer it takes for the solvent to evaporate but it still skins over in the same time it would take for a thin coat. Keep you coats at about 3 mils wet. The last thing you want to do is something that will cause it to skin over faster. There is also a lot in the archives on the subject - check it out.
From contributor R:
One thing that helps me with high solids coatings like 2k urethanes is to spray on a 50/50 thin coat of sealer as my first coat to thoroughly wet the pores and seal everything (one light pass only). This coat dries in less than five minutes and you can then scuff with a Scotch Brite pad or 320 sandpaper and spray on your full wet coat with the Kremlin. This has completely eliminated any bubbles or popping that I had.
From contributor R:
Does it happen on all pieces or just on plywood?
From the original questioner:
Thanks guys for your help and suggestions. It seems like I have already tried everything suggested, but I just can't seem to find a remedy. Contributor R, it happens on solid wood as well as plywood. I used to spray pigmented varnish on MDF and it would happen on that too. I have had this problem to happen with pre-cat lacquer and with varnish. I had trouble with the last piece that I sprayed mostly on the flat surfaces, but you could feel slight "specks" on some of the vertical surfaces too.
This has been a problem that has about driven me crazy. I’m the type of guy that doesn't mind doing whatever prep work is needed, but I really do want my finish to come out slick without the gritty feel from microbubbles (solvent pop).
I have had times where I would spray a nice wet coat on something and it would look like glass in the light reflection. I have had other times when I would spray a nice wet coat and it would look like I just sprinkled salt all over the wet piece. I try to maintain consistency with my spraying technique. That is why it’s so hard to pin down what is causing this problem. If I spray too light of a coat, the finish will look like it is starved for finish material.
From contributor R:
Airflow over the surface when it is still wet will cause solvent pop. That is the most logical reason given the info you provided.
From the original questioner:
I agree that the airflow over the wet piece may have something to do with it. I usually build my cabinets with the backs attached and so when I spray the cabinets I’m also spraying the backs at the same time. I have had these microbubbles form all over the backs of the cabinets inside the cabinets. So, it seems like airflow wouldn't be the culprit inside of the cabinets because the cabinets are laying on their backs. When I add a retarder like Flow Enhancer #2, it usually keeps the backs of the cabinets wet for a good bit of time.
From contributor S:
This may not be solvent pop. Contributor L - what does the Campbell rep say about your troubles? Do you use any other brand of solvents other than Campbell’s in your shop?
From contributor R:
Is it just on the seal coats or on the topcoat as well?
From the original questioner:
I had the Campbell reps involved quite a few times and they could not find out the problem either. I tried every imaginable thing to try to remedy it, but never could find out how to stop it. I have Kremlin 20:25 AAA pumps and MVX guns. I also have Kremlin inline paint heaters. I tried any and every fluid and air adjustment and I tried several different tip sizes and tried quite a few additive ratios, but it didn't resolve the issue. I was adding 10, 20, and even 25% of Campbell's Flow Enhancer #2 to the mix to slow the drying time down. I tried spraying with the paint booth air system on and with it off. Also, my air from my compressor is dry. I have a refrigerated air dryer and my line goes through pre and post air filters to filter the water out of the air.
When I use Campbell's products, I always use Campbell's solvents. The only difference is sometimes when I flush my lines, I may use another brand of lacquer thinner. But, it is good thinner and not cleanup thinner. If you have any other ideas, or if you know of someone I could contact about this problem, I would really appreciate any help. Also, I had been talking a good bit with a guy in the lab in Canada with Campbell. He was trying to help me with the problem. He ended up mixing up an additive for me that was not commercially available. He said to add 4 oz. per gallon of finish (If I remember correctly). Whatever it was fixed probably 80% of the problem. It wasn't a total fix, but it was definitely going in the right direction.
Contributor R – I’m not really sure if it’s been happening in the seal coat too or not. I always sand the seal coat with 220 sandpaper and so I haven't paid as much attention to the seal coat. But, it definitely has been happening in the top coats.
From contributor R:
Have you looked at the microbubbles through a 100x magnifier? That way you can tell if it is bubbles for sure or some kind of grit or contamination. Also pour a small amount of your finish right from the can onto a clean piece of glass and check carefully for grit. Now pump some through the gun onto a clean piece of glass and check again. Does the finish bubble on the glass as it dries? You should be able to at least narrow down the problem and find exactly where it is, that will give you a good start on finding an answer I hope.
