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
Mixed to 29-30 seconds
Mixed to 25-26 seconds
Mixed to 18-19 seconds
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?
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.
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.
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).
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.