Aerodynamics of Spraying

[Harvey Pascoe]

This has been a long time problem. Spraying varnish on the sides of an open box sitting on a wire mesh grid (so as not to get blowback from a solid surface), I have trouble with the spray somehow deflecting from the top edges of the box sides. No matter what I do, the spray just doesn't want to go there, and by the time I make several passes and get it covered, it will develop gravity sags above this area, the ultimate horror.

Could this have something to do with the aerodynamics of an open box that causes this since this does not happen on the closed bottom intersect with the sides. I figured this was something to do with the cheap guns I use, but now I"m not so sure. Any ideas?

[Sam Hamory]

HVLP is the answer! With a high pressure system finish doesn't always go where you want it to. Low pressure virtually eliminates those problems. I've used HVLP for over twenty years with great success and great control of my finishing.
As to the science of the aerodynamics, I'm a cabinet guy.

[Chris Fournier]

I guess that you have to take a look at your spray pattern. I would be looking for a large pattern in your situation Harvey. Also I'd have the fluid needle open pretty wide and use as little pressure as possible to get atomization. I'm looking for a soft delivery of a wet pass, hopefully one wet pass with a bit of overlap per surface.

[Harvey Pascoe]

Hah! Back to pressure again? Maybe too much? I don't thin the varnish, that's why I keep pressure high. So maybe I"ll have to thin a bit to get the pressure down. Don't get your jargon. What do you mean by "soft delivery of a wet pass"?

As for HVLP - some day.

[Harvey Pascoe]

Nope, lowering the pressure is not the answer. Well, it did get the varnish on the edges better but the spray was too weak, just dropping it 10 lbs.

[Chris Fournier]

Pressure and viscosity are bedfellows. Lower the pressure and the viscosity must change - thinner so to speak. We all have our own recipes for spraying but I would never consider spraying varnish straight out of the can, it requires too much pressure and when doing anything other than big flat panels things go south. When you can get the pressure down enough you'll see what I mean by soft I guess. Have you considered using a "jam gun" for the insides of your boxes Harvey? These tiny guns used for tight detail work are indispensible in my shop. Great for topcoats and colour work. The Devilbiss EGA is one of my favs.

[Harvey Pascoe]

My suspicion about some sort of weird aerodynamic proved correct. When I taped a piece of cardboard over the box opening, the problem ceased.

The insides of the box sides are finished before assembly, its the outside where I have the problem.

Chris, I don't like to thin varnish, first because it loses some of its gloss and second, I am spraying mostly vertical surfaces on small boxes where its very hard to see just how much is going on. Sometimes I have to hold the box in my hand and shoot it, just to be able to see. Sags are my biggest problem. In this climate, thinning just isn't necessary, but I can see where you would have no choice in much lower temperatures.

My guns are certainly not the greatest and I should look into upgrading. I'll take a look at what you suggest.

[Harvey Pascoe]

Ah, the DeVilbis 503 EGA. I actually have that gun but at the time I bought it, it was sold by Badger. It worked fine for lacquer but not for varnish which had to be thinned waaaaay down. This is practically an airbrush which covers only very small areas, too small for me. Plus, I had to clean it after every use or it would clog very easily. I can see where it would be ideal for you. Then I saw the price DeVilbis wants for that sucker $260!!!!! Who do they think they're kidding? I think I paid about $75 for the Badger version.

[Chris Fournier]

Ah, the DeVilbis 503 EGA. I actually have that gun but at the time I bought it, it was sold by Badger. It worked fine for lacquer but not for varnish which had to be thinned waaaaay down. This is practically an airbrush which covers only very small areas, too small for me. Plus, I had to clean it after every use or it would clog very easily. I can see where it would be ideal for you. Then I saw the price DeVilbis wants for that sucker $260!!!!! Who do they think they're kidding? I think I paid about $75 for the Badger version.

You can get different fluid needles and tips for the gun. I'm a sucker I guess because I paid about $180 for that little gun about 18 years ago! It has worked flawlessly so that is some comfort.

[Harvey Pascoe]

It performed very well with lacquer. My needle is pretty small and could do with larger. Do I get tips and needles from Devilbis now?

Have you ever taken that thing apart and tried to put it back together? Last time I did it took me two weeks to figure it out. Schematic was useless as that odd actuator plunger (brass) is almost impossible to tell which way it goes in.

[Chris Fournier]

It performed very well with lacquer. My needle is pretty small and could do with larger. Do I get tips and needles from Devilbis now?

Have you ever taken that thing apart and tried to put it back together? Last time I did it took me two weeks to figure it out. Schematic was useless as that odd actuator plunger (brass) is almost impossible to tell which way it goes in.

I don't know anything about the Badger that you mention Harvey so I can't speak to parts compatibility. I have had no problems with disassembly and reassembling my 503 although I did miss one washer on a JGA rebuild once that had me scratching my head for a few minutes as my gun hissed at me under pressure. I hissed back until I figured out my gaff. Doh.

[Jim Becker]

Just to add something else to this dynamic...spraying any slow-drying finish is going to raise the risk of sags, etc., in a major way over finishes that are more frequently sprayed, such as lacquer, shellac and water borne acrylics. So when you get into trouble when dealing with a coverage issue and get more finish on than will stay put, you have less leeway than you do with the faster drying materials.

Relative to the projector set, what is suitable for lacquer is unlikely to be right for oil based varnish unless it's extremely thinned. You can sometimes fudge things a little, but at some point, you need to up-size the needle and cap to what is appropriate for the viscosity of the finish you are spraying.

[Harvey Pascoe]

All my guns are needled for varnish except the Badger which is set for lacquer. But I was right about that open box effect, it does inhibit the spray from reaching the upper edge for as soon as I taped a cover on the box that stopped. Sort of like an airplane wing effect, I guess.