Trouble with Catalyzed Varnish and an Air-Assisted Airless Gun

Those pesky bubbles are back. Finishers help a colleague figure out why his new spraygun setup is not delivering the results he expected. May 24, 2006

I am pretty new to finishing and am having a heck of a rough time right now. I am using a C.A. Technologies air assisted airless set up with either .009 or .11 tips. I keep the air side between 14 and 17 lbs and the fluid between 40 and 50. My air supply is dry; I have an air dryer installed. I am spraying in an automotive spray booth with new filters. I keep the temp around 65 or so while spraying and drying.

A couple of weeks ago I was trying to shoot ML Campbell Resistant Semi-Gloss on soft maple freshly sanded to 180 and blown clean. I was getting specks in the finish. When it was wet it looked like solvent bubbles, but a lot of them. There were probably 15 or so per sq. inch. Most were still there when it dried. It looked like I sprinkled salt on the doors before spraying. The material was thinned to 26 seconds with MLC standard thinner. I tried adding 5% flow enhancer #2 and then upped it to 10%. That seemed to help but the problem was still there. I have tried shooting heavier and lighter coats. When I shot it through my gravity feed gun I did not have the problem. My theory is the pump was injecting air bubbles that could not make it to the top. But there has to be a fix.

I recently tried shooting water white vinyl sealer tinted black through the AAA on soft maple. I shot it straight out of the can and had the same problem. I thinned it and it was still there. I tried flow enhancer to no avail.

Before I tried shooting the water white vinyl sealer I was having problems with vinyl sealer but the problem seemed to be that the finish had too much texture. It wasn’t orange peel and it was more of a pin hole. They were small pin holes and close together. They were not consistent across the door. They seemed to gather closer to the applied molding.

Here are some of the things I have tried:
Flushed hose with MEK
Replaced both hoses
.009 tip
.011 tip
Mixed to 29-30 seconds
Mixed to 25-26 seconds
Mixed to 18-19 seconds

More pressure
Less pressure
Different air supply
Countless adjustments to air and fluid balance

My local MLC dealer's finish specialist is a nice guy but as green as I am I think I know more than he does. He has no real world experience and just reads from the MLC lit. I really need some help here. Does anyone have any suggestions?

Forum Responses
(Finishing Forum)
From contributor A:
It sounds like there may be an issue in regards to the fluid compression from the AAA pump. If it works well through your conventional HVLP gun then I would suggest that route for this project. Contact your dealer and ask if there are any issues related to fluid compression. I frequently have issues regarding high fluid compression when using regular airless guns with fluid pressures over 500psi.

From contributor J:
I just looked up the Resistant finish. It is a catalyzed finish. It is likely you under catalyzed or didn't catalyze the finish before spraying it.

From contributor G:
I have a couple of thoughts that may help:
To get a 26 second viscosity, it seems to me you'd need to add 40-50% thinner plus catalyst. Try warming the product and reducing it less. Use straight flow enhancer. That should allow enough time for solvent pop to disperse. Maybe your tip needs to be bigger, too.

As for the pin holes in the sealer – I would suggest the same solution. The sealer gathers near the moldings due to some physical spray property. It's deeper there and the solvent has a tougher time getting out before it flashes.

From contributor L:
I'm sort of with Axel on this one. Get past this project first using your gravity feed, then go about trouble shooting your other system when the stress of this job is finished. I know it's hard to do when you have a new toy, but if you want to reduce the chance of having a heart attack, take the easy route first.

If you want to be stubborn, try to mist coat as quickly as you can across the surface and on the return pass, slow down and create a coat that looks wet but not heavy. As for the solvent pop, (bubble burst) you should have no problem eliminating them with additional finish. The pin holes may need to be misted out over several passes (coats).

From contributor S:
I have had the same problems with Resistant over the years but hit or miss, it would go well for a while then pop up again. I was getting it with my AAA and my pot, and the flow enhancer seemed to make it worse and in my case they never solved the problem. Since I have found Becker Acroma I don't use their Clawlock/Resistant any more.

From contributor T:
I've had the same problem with Kremlin in very dry conditions. I was only able to get rid of it by switching to a .004 or .006 fluid tip, thinning to about 17/18 seconds (Z#2) and keeping the first couple of coats very light (2 mil wet). It’s more work, but no bubbles.

From contributor D:
Get your C.A. Technologies tech support involved. They are a fine company. They make a superb product that you have. Make your problem their problem because that's what customer service is all about.

From contributor C:
Of course you are stirring your material and not shaking it?

From contributor R:
Try adding a little butyl cellusolve to retard the drying time, allowing for the finish to flash before the bubbles form. This can happen when the humidity level is very low. Also, put the freshly sprayed parts in a place where there is no air movement. If you are racking them in the spray booth, the air movement over the freshly sprayed piece can cause solvent pop.

From contributor R:
I forgot to mention - we shoot precat lacquer and CV from Sherwin Williams with our Graco AAA all the time and have never had this problem. I have had this problem before with MLC.

From contributor E:
I am with Contributor S. Find your local Becker Acroma distributor and get some of their 212 or 318 waterborne. They are just incredible and way safer and easier to clean your equipment. I think their toughness and chemical resistance are right up there with post-cats, maybe not up to conversion varnish but close enough.

From contributor W:
What you've described is small solvent bubbles that are trapped below the surface are held there when the finish skins over. Many times some of these bubbles will just break a semi-skinned surface and leave a small crater. It may well be that the solvents used by MLC are sensitive to some conditions and will prematurely skin over. Almost any lacquer will if the part is cold and the lacquer is warm, if it's too warm or too dry in the booth, or if there's too much air flow passing over the part as it's flashing off.

It is also possible that the solvent used for thinning is not fully compatible with the coating product - it doesn't evaporate before the lacquer skins over. Some folks believe that any old lacquer thinner will do but it is really best to stay with the mfr's recommendation since there are over 20 possible constituents in "lacquer thinner". They tout their fast drying time, no HAPs and low VOC's and I'm sure have very carefully selected their solvents. It sounds like you've used the right stuff but you may want to double check the recommendation for Resistant.

Occasionally these bubbles will develop if a second coat of material is applied before the first has fully flashed. (Always let the last coat flash off before putting on another.) But the most frequent cause in my experience is piling on too much finish. When the coating is too thick the solvents just can't get to the surface before the stuff starts to skin over. If you're using a .009 or a .011 tip you're laying on a lot of material unless you're very fast with that spray gun (and you may be). Your gravity gun probably has a much lower fluid delivery rate than your AAA with the tips you're using which would explain why it worked better. You say you tried lighter coats but check your wet mil thickness: I would keep it below 4 mils wet. If it is over 4 mils, switch to a smaller fluid tip.

I'm curious to know if you tried retarder in a un-thinned load. When you thin it you've got two problems: one, there's more solvent to evaporate which takes more time, and two, your gun is shooting more material. I would be surprised if the problem is being caused by your AAA but the issue comes up a lot.

From contributor W:
Sorry I forgot: you also want to be sure you're not over catalyzed which will most likely shorten the time it takes to skin over.

From contributor K:
I have sprayed hundreds of gallons of Resistant with a Kremlin and it all came out slick.

From contributor B:
To contributor K: What size tip do you use for the Resistant?

From contributor K:
I prefer to use a #11 tip but have sprayed it with a #9 as well. I just have to slow down a bit with the #9.

From contributor J:
I catalyze 10% and use 20% #2 Flow enhancer and it works perfect for me every time.

From contributor H:
I would suggest trying your fluid pressure at 30 and air pressure at 11.

From the original questioner:
I took another shot at the MLC white vinyl sealer tinted black and it went better. I tried a number of combinations but what worked best was spraying a very light coat of straight vinyl, letting it dry an hour, scuffed with 180 and then I sprayed a good wet coat of about 3 mils. It came out looking really good. I would like to come up with a way of doing it with a single coat and I plan to work on that some more. I also want to give resistant another try before giving up on it with an AAA.