Air compressor questions

Any questions about tools or supplies. Post your compressor/gun questions here.

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PostPosted: Wed May 06, 2020 8:49 am
Total newbie to gravity gun auto painting so I need some direction in the air compressor dept.

1. How do you maintain the CFM if you are constantly using air while spraying?

2. I have a 21 gallon 3hp compressor rated @ 7 SCFM @ 40PSI: can I paint small cars with this or do I have to paint the car in sections only and why? Looking at a 4-6 CFM rated gun

3. Where is the PSI regulated, at the compressor itself, or the base of the gun or both?

Also my compressor is 1/4 NPT; should I convert it to 3/8?

a million thanks in advance, Doug

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PostPosted: Wed May 06, 2020 10:50 am
The air compressor has a rating that should be listed on a sticker or tag:
Air Compressor 3.JPG


The displacement is what the compressor puts out with no load.
The CFM or SCFM is the volume of air delivered at a set pressure PSI.

It is the air delivery numbers that are essential to the spray gun working properly. A larger tank will hold a reserve of air that will last a short time before the compressor has to kick on to maintain the CFM delivery.

As for the air supply system, you will have a regulator on the compressor that is usually set fairly high. Mine will go to 175 PSI but I set it a 150 PSI just so it doesn't have to work so hard.

From the compressor you will want at least 20' of copper piping (I used 3/4" on mine) to cool the air before it reaches your water trap and filtration system. Most water traps have another regulator and are mounted inside the paint booth. This regulator will be set usually around 100 PSI.

Any restrictions, like 1/4" fittings, will reduce the volume of air reaching your spray gun. Hence most use 3/8" diameter air hoses and hi-flow fittings.

Do a search for Air Supply Systems and similar and you should come up with more detailed information and diagrams.

Also, the Info Center has good information for beginners. Here's a link to get you started: https://www.autobody101.com/content/art ... ould-know/
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PostPosted: Wed May 06, 2020 6:51 pm
'68 Coronet R/T wrote:The air compressor has a rating that should be listed on a sticker or tag:
Air Compressor 3.JPG


The displacement is what the compressor puts out with no load.
The CFM or SCFM is the volume of air delivered at a set pressure PSI.

It is the air delivery numbers that are essential to the spray gun working properly. A larger tank will hold a reserve of air that will last a short time before the compressor has to kick on to maintain the CFM delivery.

As for the air supply system, you will have a regulator on the compressor that is usually set fairly high. Mine will go to 175 PSI but I set it a 150 PSI just so it doesn't have to work so hard.

From the compressor you will want at least 20' of copper piping (I used 3/4" on mine) to cool the air before it reaches your water trap and filtration system. Most water traps have another regulator and are mounted inside the paint booth. This regulator will be set usually around 100 PSI.

Any restrictions, like 1/4" fittings, will reduce the volume of air reaching your spray gun. Hence most use 3/8" diameter air hoses and hi-flow fittings.

Do a search for Air Supply Systems and similar and you should come up with more detailed information and diagrams.

Also, the Info Center has good information for beginners. Here's a link to get you started: https://www.autobody101.com/content/art ... ould-know/


Thanks for the reply. I live in South Florida. I wonder if the copper tubing will make the air hotter, as my garage can reach triple digits temps at times.

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PostPosted: Wed May 06, 2020 10:40 pm
A small compressor trying to "keep up" by running almost continuously is what develops heat. Running copper starts to let the air cool down which starts making moisture in your lines, then spitting out of your gun. A nice big 2 stage compressor, 3/4" lines of good length, full flow fittings and proper moisture traps/eliminators is what it takes for good air supply to guns. When you skimp on any of that something is going to take a "bite" out of your paint job.....
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PostPosted: Thu May 07, 2020 8:47 am
DarrelK wrote:A small compressor trying to "keep up" by running almost continuously is what develops heat. Running copper starts to let the air cool down which starts making moisture in your lines, then spitting out of your gun. A nice big 2 stage compressor, 3/4" lines of good length, full flow fittings and proper moisture traps/eliminators is what it takes for good air supply to guns. When you skimp on any of that something is going to take a "bite" out of your paint job.....


So you don't recommend copper tubing?

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PostPosted: Thu May 07, 2020 9:13 am
No, copper tubing is fine. By doing longer runs of it or making a looping grid of it with an auto or manual drain this allows you to get the water out before it reaches your tools/guns. The latest, greatest stuff to work with is the RapidAir system. I just ran a bunch of this myself in my wood restoration shop. That comes in "spools" that you bend out and shape. I think we ended up running 140 to 150 feet of it when done. If you are just a hobbyist you could get by with 1/2 inch stuff. We went with the MaxAir 3/4" size.
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PostPosted: Tue May 12, 2020 1:48 pm
As far as your compressor is concerned - what you have is minimal at best for spraying automotive paint. If you are serious about what you plan to do in the future and want to eliminate frustrations, I highly suggest you purchase a unit that will put out MORE than what you require. Remember that the compressor is the heart of your shop.

visit my diy work at www.hotrodreverend.com. I am no professional, and up to the work I completed on my 1955 Ford, I had never sprayed professional automotive paint. The advice given on this website by such experienced fellas in the field will be a big help to you - it was to me and I am very happy with the results!
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