How to Prevent "Cobwebbing" of Pre-Catalyzed Finishes
It's a matter of finding the right thinning ratios and air pressures for the particular gun — and using the right thinner. February 13, 2006
Question
I recently began experimenting with precat lacquer but I get white, hair-like particles each and every time I spray. It is not overspray. It is visible as soon as I stop spraying the very first piece. I am using an HVLP spray system and have experimented trying different pressure, volume, thinning, etc. Any ideas?
Forum Responses
(Finishing Forum)
From Paul Snyder, forum technical advisor:
This sounds like cobwebbing. The finish enters the air-stream at the air-cap and dries in fine strands that end up on the surface you're spraying. It can be caused by an imbalanced lacquer thinner (not right for the lacquer), lacquer that is too thick or cold, or air pressure that is up too high. If you have to crank up the air to get the lacquer to spray and atomize, try thinning it a little.
From the original questioner:
I just increased the thinning to 15% and it seems to help. I first tried 5% then 10%. Also, I am a rookie with this HVLP and am not sure what pressure I should be operating at. The manual is limited. I find that between 35-39 psi seems to atomize it the best. Is this too high of pressure for HVLP?
From Paul Snyder, forum technical advisor:
Every spray gun is different Bo. There's no set pressure that's right for all of them. Basically, you want to use the least amount of air possible to get good atomization and fluid flow. If the lacquer is coming out of the gun kind of slow (fluid flow is low), try thinning some more. I had one spray gun that only worked well when the finish was thinned almost 50%.
Make sure you're using a good quality lacquer thinner. Some of the cheap ones have too much diluting solvents (diluents) in them and will kick the lacquer out of solution and cause the white strings or clumps. A little slow evaporating lacquer thinner (retarder) will also help. Your supplier should have both the regular lacquer thinner and retarder along with a recommendation for how much retarder to use. Usually it's somewhere in the 2% - 10% range, and you want to use the least amount possible.
From contributor A:
I definitely agree on the use of the appropriate thinner. I've not dealt with cobwebbing before, but learned the miserably hard way that cheap thinner will cause solvent pop. We use ML Campbell's Standard lacquer thinner which produces great results for us. $60 buys a 5 gallon can, vs. $30 for the cheap cleaning grade. We only use the expensive stuff for thinning - we use the cheap stuff for cleaning only. And yes, precat is the bomb. There are significant differences in quality though, between and even within brands. We only use MLC's Magnamax, and we've tried Magnalac which is their lower-end precat. We did not like the Magnalac. Solids content seems to be very low for a precat.
From contributor B:
Just to add to what Paul advises, use the lacquer thinner recommended by the tech sheet for your topcoat. It is formulated to be a balanced blend of solvents that you need for reducing your topcoat. Do not be shy about thinning down your coating. You need to get to a spraying viscosity that your gun can handle. To optimize your spray gun turn your air pressure down to about 5 psi. Open up your fan control, not all the way, but enough so that it is set to about where you would get the maximum width fan or just shy of that setting. Now, adjust the position on your air cap so that the fan is horizontal - the horns should be positioned vertically and the horizontal fan would be what you would be using if you were spraying a piece of molding that was standing straight up.
Spray a short burst of spray on a surface that is wide enough to accommodate that burst so that you can examine the quality of your fan. Keep incrementing the amount of psi that you give to each experimental burst. You are looking for a fan that seems to develop drips or runs that are even across the width of the pattern, like the squeeze out pattern on properly glued mating pieces of wood.
Increase your atomizing air in 3 - 5 psi increases. Keep increasing the psi until your pattern seems to get no longer in length. Cut back a few psi. Your spraying psi is now optimized for the material you are spraying. Reposition the air cap so that you can now begin spraying as normal - unless you need that fan position.
Bob Flexner summed this all up so much better than what I have done and it’s in his newly revised edition of *Understanding Wood Finishing*. By the way, if you are using a remote pot, set the pressure for the pot so that your stream of unatomized fluid shoots from the gun about 8''- a pinky-to-thumb measurement - before it begins to drop off. That will give you decent fluid flow rate.
Generally, from precat to precat you may find that you need to reduce as much as 40% to get a viscosity of material that you can work with. Use the needle, nozzle and aircap that is rated for the viscosity of the material that you want to use.
From the original questioner:
I had used store bought lacquer thinner. I went back to the paint store and they indeed sold an exclusive lacquer thinner specifically for my precat. It made a big difference. Also, I apparently was not thinning it enough. It took a 35% thinning for it to spray as you had described. I was reluctant to add thinner. I was able to reduce the air pressure quite a bit and that reduced overspray to a minimum. The end result was that I think the side tables look great. Next project is a set of custom birch cabinets.
