Sand-Like Imperfections in a Conversion Varnish Finish

Finishers troubleshoot an unusual flaw. November 26, 2007

I only recently started using CV and ran into an issue today. When I'd lay down a coat, it looked like the piece was covered with dust. I blew everything off with compressed air, and wiped it down with a tack cloth, but this didn't cure it. It was extremely humid and drizzling out, and I don't know if this was affecting the finish. I have a Devilbiss extractor (which was picking up a lot of water) and a Motorguard filter on my Kremlin unit, so I would think those would keep contaminants out of the air supply. The inline fluid filter keeps the particles out of the fluid. It's not solvent pop. Does anyone know what's causing this?

Forum Responses
(Finishing Forum)
From contributor M:
Sounds like a blush problem from humidity. Maybe get some retarder or a slower thinner from your supplier.

From contributor R:
My guess is the same as contributor M's. Contact the manufacturer of your CV and explain the issue to them. You may not want to apply the coating in such a drastic environment. Adhesion problems can creep up and the gremlins of finish failure have no mercy.

From contributor O:
I don't think blush is a problem as with lacquer. I have been using CV for many years and haven't seen this occur. What happens to the finish when it is dry? Why don't you think it is solvent pop? Did you apply a sealer first and sand it?

From contributor A:
Eliminate the use of a tack cloth and see what happens. Also, there are times that a coat of CV may look grainy, but when dry, levels out pretty nice. Too, if you are using 275 VOC, solvent pop may be the problem, and a crazy maker. If so and if possible, bump up to 550.

From contributor J:
I ran into a similar problem a while back. It seemed that no matter what we did we could not solve it. Every time I would spray a piece, it looked like sand in the finish. We finally found out it was the flattening paste in the finish and that we were not shaking the buckets long enough. It can take 10-15 minutes in a shaker to break all of that stuff up. The way we finally caught it was to strain our finish with a cotton rag over a mixing cup and see what it catches. If it catches what looks like little blobs of jelly, then you have flattening paste issues.

From the original questioner:
I tried everything today, and it's not moisture. I added a water filter after the 2 filters already in place and it didn't show a drip of water after a morning of spraying. Also, the humidity was much lower today than yesterday. The problem was as bad today as yesterday, though.

It seemed like two things were happening. Most of the specks seemed to be bubbles that appeared immediately when the coat was laid down. It wasn't solvent pop, as it happened right away. The best results I got were by turning the atomizing air down some, and increasing the fluid pressure, holding the gun further away, and speeding up my passes. That made it better, but didn't cure it.

The other thing that was happening really showed up when I increased the atomizing pressure to the extreme. It seemed like little flecks of finish were forming before the material hit the surface. I took the gun apart, cleaned it, and did notice that a small bit of material had crept into the air chamber of the gun. This may have caused some of these material flecks, but it seemed like the very high atomization (and bad tip?) were the culprit.

So I still need a cure. I did a search on "air entrapment," and several people recommended Kremlin's ultra tip to eliminate the issue. Is it possible that a bad tip is causing all this to happen?

From contributor I:
Is this a pigmented CV or clear? If pigmented, whose pigment is it? Also, what size tip are you using for the Kremlin?

From the original questioner:
Clear (Kemvar) with an 09 tip.

From contributor E:
It's possible that you are pulling air into your paint from a small hole or crack in your intake fluid hose. Micro bubbles can form and they look like little dust particles. I always like to spray out my finish on glass to eliminate gassing off from the wood. I can't say I have had a tip cause bubbles, but that doesn't mean somebody else hasn't.

From contributor I:
The reason I asked about pigmented and tip size is that there was a problem called "shearing" with a certain pigmented lacquer running through the Kremlin. With the action of the pump and air pressure, it caused bubbles in the finish. The solution was to change tips and lower the pressure. As the previous response said, it also could be air intake from a pinhole in line or loose clamp connection. As far as proper air/fluid pressure is concerned, I found that if I put the right gauge around 10-1030 position and sprayed, you would find on your spray pattern at each end a definite line. These lines are called "rooster tails." Then on the left gauge, increase pressure until these lines disappear. So regardless of the type or reduction of product sprayed, you should have the correct lay out for that product... Works for me.

From contributor G:
Rooster tails?

Anyhow, here's the archive on micro bubbles...

Pump Action Causing Micro-Bubbles in Polyurethane Finish

From contributor I:
Yep, rooster tails... And that is on a 1019 pump. The larger pumps are different, I understand, but I don't know, as I don't use them.

From the original questioner:
Thanks for all the feedback. I had ruled out a leak in the fluid line, as I thought that any bubbles that formed in the liquid before it was atomized would be popped during atomization. But that may not be the case. My fluid line T's off to 2 separate intake areas, so there are a number of joints. Any suggestions on how to check for an air leak? Also, how much are people generally thinning the Kemvar?

From contributor N:
When you sand CV, you have to wipe the dust with a cloth. Then blow it off. Might be it is under-catalyzed. That would make it appear that way. Does it dry well?

From contributor U:
We have had the same problem numerous times, and almost always tracked it back to old CV. Check the date on your can - you may have to call the manufacturer to tell you how to read the code. Anything approaching 1 year old is sure to do this. We call it micro seeding. Our theory is that the CV has started to self-catalyze and form those very tiny hard bits, like very fine sand. They will always be evenly dispersed across the surface. Often, if the particles are very small, they will disappear overnight, as the CV coat shrinks down. We've learned not to panic when we see this stuff - and to check it again the following day.