Air bubbles in conversion varnish

Why are they appearing when finish is sprayed on with an airmix pump? March 23, 2002

We are using a Kremlin airmix 10.14 pump and for the most part are very satisfied with the results. But one problem keeps coming up. When spraying cabinet doors flat on a bench, we notice small air bubbles in the finish when wet. Most lay down when it flashes off, but some are still noticeable when dry. We use Sherwin Williams CV reduced 15% with xylene, an 06 tip, and air and fluid pressure at 10 and 50. We have tried different pressure settings with the same results. Any ideas?

Forum Responses
I shoot a lot of SW Con-Var. I have the same problem. I have had better results if I cut the first coat by 50% with xylene or, in warm weather, with High Flash Naptha (to slow flash time). Second: a lighter first coat (spit coat) followed by a full sanding. 220 grit also helps.

I'm not familiar with the SW conversion varnish, but most CV's are made to have a high solid content. With the Kremlin system, you might want to try the 09 tip instead. It has a larger orifice than the 06 tip. The 09 doesn't come with a screen inside the tip, but it seems to work better for higher solid materials. The air into the machine from the compressor should be around 75 to 80 psi. Then set the fluid pressure to where you get about a 3 foot stream shooting out, then adjust the air pressure up. When looking at the gauges, set them at a 10 o'clock position, on both the fluid and atomizing air pressures. You may need to do some adjustments from there up or down, but this is a good starting point.

Lisa Gilbert, forum technical advisor

This is more of a coating issue. It sounds like you're getting solvent pop. That Kremlin's putting on twice the coating in one pass than a pot system would. Try backing off a bit, Plus, with the Kremlin, there's really no need to add solvent to your coating. I'm not familiar with SW CV, but my guess is that the solid contents are 40-45%. Your pump pressure (red) should be around 30-35 psi and your air (grey) should be 12-15 psi and I also would recommend the 09.096 tip.

Lisa, what do you mean by a three foot stream? I believe you are getting your atomizing technologies confused. The stream will be from an air or hvlp gun, not an airless type spray, which will have a fan or conical shaped pattern.

One other thought regarding a 6 tip. You can remove the tip filter screen and go with a higher mesh gun handle filter.

I have one of these systems in my shop. The 10:14 Pump System referred to is an Air Assisted Airmix unit. It is equipped with a 5 gallon siphon (pick-up) tube, so I get my paint right out of the bucket and have the #09-096 tip, just for spraying high solid finishes. I use this system with a 1.5 hp portable air compressor. The system has dual controls - one for fluid (red) and one for air (grey). I can check the fluid without the air being on by turning up the fluid pressure until I have the 3 foot stream that I need.

I use the same unit when I spray pre-cat or conversion varnish products. Only difference is I use other name brand products. Not only do I have one of the systems, I have sold this same system to quite a few cabinet shops in my area. I teach each shop how to set up and maintain their systems the same way.

As for taking the screen out of the #06 tip, it will still have a smaller orifice than the #09 tip. To me, with the higher solids of CV's, it's like trying to squeeze an orange through a straw. You can change the screen at the end of the gun to help strain the paint before it goes through the gun.

Lisa Gilbert, forum technical advisor

I think Lisa'a description of the "stream" out of the tip occurs when fluid is pumped through the gun with the trigger pulled and the air cap off.

From the original questioner:
I'll try all suggestions. Although we have no problem spraying with the 06 tip, I'll try the 09 and play with the reduction of the CV. The problem is we don't get the air bubbles all the time, and when we don't, we can't figure out what we did differently to eliminate them. So that leads me to think that it might be a wet film thickness issue. Even still, I won't trade the Kremlin for anything.

From contributor R:
What species of wood are we talking about on these cabinet doors? Oak? Ash? I'll bet you don't have the problem on maple doors.

From the original questioner:
I'm familiar with the bubbles common when finishing open grain wood like oak and ash, but the bubbles I'm getting are very small and on closed grain wood like cherry and maple.

From contributor R:
Do you use an agitator? If not, how do you mix your material before spraying?

From the original questioner:
We do use an agitator. I didn't think of that as a possibility. Although, today we sprayed some cabinets and reduced the CV with hi flash naptha only, spayed a 40 percent reduced tack coat, then seal coat, sand 320, and a 15 percent topcoat, and it came out great. We will try it again on the next job and see if we just got lucky.

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

Comment from contributor A:
I work for a large custom cabinet manufacturer, and we have about 35 or 40 Kremlin Airmix guns (automatic and handspray), as well as other brands. The problem you describe is air entrapment or air entrainment. It results from the microfoam that is generated during the atomization process at the tip. Air-assisted airless is particularly troublesome for this; we sometimes experience it too.

Things to look for include keeping the atomizing air as low as possible, just enough to break up the "tails" seen with no air. Also, be sure that your substrate is at the proper temperature. Atmospheric conditions have much to do with this problem, especially hot, dry air which promotes rapid surface-cure; this is why slowing the coating helps. I am concerned that cutting solids as much as I've seen recommended here will rob build. I'd be more comfortable with a smaller add of a slower solvent such as Butyl Cellosolve, n-butanol or isobutanol, or even 2-ethoxyethylpropionate (EEP). A few ounces of any of these will slow the dry and should help. Obviously, cutting the solids drastically works, too.

I am a chemical engineer, and worked for paint manufacturers for the first 13 years of my career, and now 7 on the user side, and have battled this issue many times. You may also want to inquire of your SW rep if they have any kind of bubble breaker available, but be cautious with these. They are, by their very nature, somewhat incompatible and in some instances may cause or intensify a fisheye problem.

Comment from contributor B:
I am the foreman and lead sprayer for a small custom cabinet manufacturer, and our spray department likewise uses the Kremlin Airmix 10-14 pump with the Kremlin Airmix MX 120 LT spray gun using the 09.094 tip. We noticed the same problem you are experiencing because.

We recently expanded our operations into a new, larger industrial complex with a completely new spray department installed, all new equipment. However, immediately after beginning operating there, I noticed that some of the finished product was exhibiting the same small air bubbles you described. My first thought was air entrainment, and this evidently was indeed the culprit. The problem was the final coat of (pre-cat) lacquer was flashing off too soon - as the gasses released, forming the bubbles, the material wouldn't settle out.

I believe this could be what is causing your problem too, from the way you described it. At any rate, it's something to check for and is certainly an uncomplicated solution. I also concur with Lisa Gilbert - you might be better off with the 09 tip for final coat, which is what I use (09.094 for the wider fan width). For the pump settings, I set the gun air regulator (grey knob) to 15-20 psi and the fluid air regulator (red knob) to 50-55 psi.

Comment from contributor C:

Trapped air bubbles can be caused by a few different things such as high humidity, high temperature, too thick of a coat especially close to the edge, or air in your line. First don't use a 06 tip to spray lacquer use the 09 096, 09 114, or 09 154 air between 15 and 20 psi fluid between 50 and 60 psi. Spray at a 90 degree angle 5 to 6 inches from the panel and overlap your pass at 50%.

The first thing to check when you have trapped air bubbles is if you have air in your line. Sometimes that happens when you are getting low in product and then your pump is pumping air and then you add more lacquer. If you don't flush your line until the air is all out you will always have air in your line.

To know when the air is all out is simple. When the lacquer is coming out of the gun (without the air cap on) and it is clear then you are good to go. If it's a really humid day or a really hot day I would recommend to add about 5% eep or buttylcellusove into your lacquer that will slow it down and that will give the solvent time to escape before the top of the lacquer starts to dry. Also, don't forget to check your wet mill once and a while. It should be between 4 and 5 mills. If you are checking all those things you should not have any more problems.

Comment from contributor D:
I am a coatings formulator and have produced water, UV, and CV for plastics, metal, and wood for 27 years. When it comes to spraying using an Air Assist airless for producing entrained air nothing is the least optimum choice. If you can tolerate some slower solvent this can help, but a good air release agent will do a lot to knock down that entrained air as soon as possible before the visc. climbs and flow decreases. Good wetting of the cellulose of the wood and for paper can be improved with substrate wetting additives. Experience is the best teacher. If I was not forced to use an Air Assist, and permitting issues, I would spray conventional air atomized all day long. This gives you the most control over your coating.