Gun: Devilbiss FLG 670
Compressor: 5 HP (actual), 60 gallon, 15 cfm at 90 psi
Spray PSI: 23 psi (according to the Devilbiss gauge)
Air Lines: 1/2" hard lines, leading up to 1/2" ID soft line and high flow fittings throughout
Filters: Campbell Hausfeld, flowing plenty of CFM
Paint: Eastwood SSU Pinup Red
Skill Level: Novice

After having prepped fenders on a 55 Ford Fairlane to make them laser straight, I am very, very pleased with the results of the finish. I will post photos as soon as I can. I set up a paint booth made of PVC and 5 mil plastic, with box fans to bring air in and out. The initial replacement of room air was unbelievable during the first coat and mostly into the second coat on the fenders. In the middle of my second coat, the exhaust filter clogged up big time (only a 20x20 filter) and then the intake fan kept blowing air into the paint booth which made the overspray find any and all exit holes from my make shift booth.

The question I have is this - how much is "normal" to expect concerning overspray from an HVLP gun? I got to tell ya, it seemed like the overspray was just as much as a conventional gun and I am not sure why. The volume of air is excellent, the regulator and gauge at the gun (supplied from Devilbiss) is correct I assume. The hard line runs about 40 feet before hitting a filter and regulator, and then about 30 feet of soft line to the gun itself.

This was the first time I sprayed with this gun, but again, I like the results of the finish - no runs, good flow and coverage, etc. Just kicking out a lot of overspray it seems to me.

Should I dial back the psi at the gun itself next time? The volume of air seems excellent, as does my "cigar shaped" pattern for material flow.

Edit: Photos added

i'm with you ,I have a Sata HVLP gravity gun and several Devilbiss guns conventionals MBC, JGA. I think I get less overspray with the coventionals than with the gravity HVLP. one thing to do with any gun is to regulate you air pressure, you want just enough pressure to atomize the paint properly and no more. if your paint wiil spray properly at 30 psi at the gun and your using 40 psi you going to get a lot of unnecessary overspray and wasted paint. it takes a little experience and will vary sometimes with color, type and reduction. before stating a paint project I always shoot a test panel. if i'm getting OP i'll turn up the air just until it gos on smooth.
Jay D.

Sounds to me like the problem is not your gun or settings but rather your booth.
Typical airflow rates for a cross draft booth run 100 ft per minute but even half of that would clear over spray out of your booth quickly.
A 20 x 20 filter is not adequate to remove over spray as can be seen by how quickly it plugged up. To give you an example I have 20 of those filters in my filter bank.

Try adding another exhaust filter of two and see if that doesn't help considerably.

^^^ More air flow.

I have a hombrew 10'X10' with 3 fans blowing filtered air in and basically unlimited exhaust through all the cracks in the floor of the deck and leaks in the enclosure. Still gets foggy inside quick if I am spraying with a full size gun. But dissipates fairly quick too.

Fellas, thanks for the comments, but I do realize the issue of not moving air, small filter, etc. The question I have is about why there is so much overspray on an HVLP gun. It has been years since I have sprayed with a conventional gun, but as I recall the overspray from a conventional, the Devilbiss here is matching the volume of overspray.

I only put three coats on two fenders, and a tutone job at that, so not a lot of surface area. There is much, much more overspray than when I spray my DTM Tamco Primer Surfacer through a HF gun...

I'd do agree with the guys on the homemade booth thing but I don't think you can exactly "quantify" the overspray issue concerning HVLP. I worked with gun manufacturers (turbine system builders) during the early 80's era which actually triggered (pun intended) the trend starting in southern California and resulting in the first HVLP compressed air gun designs which the EPA then adopted. HVLP by definition only meets basically one requirement.... 10 p.s.i. at the air cap. Back in the good ol' days 35% of what was in your quart cup ended up on the surface of what you were shooting. By doing that 10 p.s.i. rule at the cap, transfer efficiency jumped to a minimum of 65%. Was there a reduction of overspray?... well, of course. Was there any standards established for a set amount of reduction?.... uh, no. I've found that there is a rather wide variation in overspray from gun model to gun model. My current wood shop 4 stage turbine 5 gallon pot system is up around a 96% transfer rate. You can put your hand next to a 8 inch wide 4 mil wet cigar I'm laying down and get almost nothing on your hand. I'm just shooting another HVLP gun, right? HVLP.... there just letters.....

Thanks Darrel, that makes sense for sure.

I did notice on a few youtube videos that most guys are turning down the regulator to something like 15 or 17 psi to shoot paint from this gun. The video shows just about no to little overspray. That is probably what I will do next, get in the booth and fiddle around until I find something that works to keep the overspray a little more manageable. Of course, I know my makeshift booth does not help, but your explanation on the differences among HVLP guns makes quite a bit of sense as to how much overspray each one will actually put off into the air instead of on your panel (transfer efficiency right?).

Yep, transfer efficiency is what the big mandate was all about. And, yes, that's the next thing to work on with that gun....tweeking. Air pressure, gun settings, material viscosity, room temp.s, etc., all play into "how" stuff comes out of that gun.
I just went through this setting up an HVLP compressed gun for the application of heated glue based materials. Thing was an overspray hog when I started. Pressure reduction at the gun plus actually increasing the fluid viscosity got us better application with much less overspray.

Darrel, have you tried the 3M accuspray gun? With my limited use of it, I think its awesome.

what was the booth setup years ago when you used a conventional gun?
maybe there is a difference there?

just going from a devilbiis startingline gun to a tekna my over spray increased. I was told it was owed to the better atomization of the tekna.