Compressor and Gun Air Pressure Settings for an HVLP Spraygun

      Set the compressor a little high and the gun a little low, then dial it in with test boards ó here's how. April 19, 2011

Question
I have a craftsman air compressor I believe it is; 1.5 HP, 26 gallon, output 9.1scfm at 40 psi, 7.1cfm at 90 psi. I have a new husky Pro HVLP spray gun (not that great) with a 0.047 fluid tip, scfm 6.5 avg. at 40psi, max inlet pressure 80psi, HVLP inlet air pressure 40 psi.

What PSI should I have my air compressor set at before I start fine tuning the gun?Any tips on fine tuning the gun would also be great. I will be spraying lacquer.

Forum Responses
(Finishing Forum)
From contributor F:
I have a regulator at my compressor set to 110psi and this feeds the shop. Right at the gun I have a little restrictor/regulator with a gauge that I use to adjust gun pressure.



From contributor U:
I spray with either a Binks or Devilbiss cup gun, HVLP, and I normally have the pressure set around 60# coming out of my filters and regulator in the spray room. When spraying lacquer sealer, I back the pressure down to 55 lbs. When spraying my topcoat which is a little thinner, I either stay the same or back down a few more lbs to 50 -52. I am not sure of the pressure at the gun, but I use a 3/8" hose 25' in length.

I spray keeping the pressure high enough to of course move the lacquer nicely and fast enough to move the work on through. But I keep the pressure low enough to minimize the overspray fog that will make for a rough finish when it settles on the product. A good fan moving lots of air and adding retarder keeps the overspray down, but watching my pressure helps a bunch too.

When spraying Sherwin Williams cab acrylic lacquer, medium rubbed effect, I add 2 oz. of thinner, 1 1/2 to 2 oz. of retarder and fill the quart cup up the rest of the way with lacquer. Stir really well before spraying. I spray only clear finishes in lacquer, so some adjustment may be needed for paints.



From the original questioner:
Contributor F - what psi at the gun do you suggest for lacquer?


From contributor J:
Try this: open the fluid and fan controls on the gun all the way. Turn the air pressure all the way down and pull the trigger while slowly dialing up the pressure to the point where atomization is fine enough not to be mottled or spitting but no so high that it is blasting more air than you need. Try spraying some test pieces at various pressures and note the results of the finish when dry. You may be surprised at how little air pressure you really need to get satisfactory results. I suspect your sweet spot will be about 35 lbs. of inlet pressure.


From contributor F:
Depends on the temp, but I am usually around 35 to 40psi give or take at the gun. This drops to around 18 or so with the trigger pulled. Contributor J gives a good tip on setting the pressure.

Take a piece of cardboard or something and start dialing the pressure up as you take really short bursts on to the cardboard. You will reach a point where you have nice even droplets (good atomization). Once you reach this point, you can typically back the pressure off just a tad to reduce overspray. Now spray a test piece. Judge it about a minute after you spray it to give it a chance to level out. The goal is to find the lowest possible pressure where you get a nice finish.

Sometimes you will find with the pressure a little low, you donít get a glass-like finish immediately after spraying, but after a minute or so, it will flow out and be nice and level. There is no one correct pressure setting. Outside temp as well as fluid temp play a role in the correct gun settings. If itís really hot outside, your part may flash really fast so the fluid wonít have enough time to level. If itís cold, the fluid will be thicker. You can add thinner to reduce the viscosity, or turn the air up a little to get the atomization you require.

It seems a lot more difficult than it actually is. Set your gun at say 30lbs and start working up a little till it looks good. After spraying a few times with different weather conditions, you will get the hang of it really fast. You should also have a couple different tip sets for different products. Thin stuff like NGR's will spray nice with a 0.8mm tip, typical lacquers and CV's will be good somewhere in the 1.3 to 1.5mm range, and primers and thicker stuff will like something around 1.9mm. CAT makes some nice sets that come with a couple aircaps and a few tip sets that work well with typical wood finishing products. Somewhere around 35 psi at the gun will be in the ballpark.



From contributor C:
Every gun is different. You need the specs for your gun. I have a HVLP that specs18 PSI and another that calls for 45 PSI. That is a spec measured at the gun with the trigger pulled all the way back. Most HVLP's won't get enough CFM's if you're using standard air fittings, they're too restrictive. You can overcome that by running 60 PSI or higher to the gun and then regulating down to the gun spec.



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