Question
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.
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.
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?
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.
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.
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.
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.
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.
