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Old 08-06-2004, 06:50 AM
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Paint gun / Air pressure question

Two buddies I know both paint. In fact one is shooting my truck right now. The last that I painted I used acrylic enamel with a siphon feed gun and around 50 psi at the regulator. My two buddies both use HVLP and are running at 80 psi at the regulator. Now I thought that with HVLP you could run less line pressure than with a siphon feed gun hence the LP and the qravity feed gun was designed to push out more volume hence the HV. Am I correct in my thinking or am I thinking it backwards? I have yet to hear from anyone around here using an HVLP running 30-40 psi line pressure. I do know that when I shot using a siphon feed with an average 50 psi I could lay down a super smooth finish with high gloss and very little if no peel.Both guys using the HVLP guns lay down a smooth coat also but the pressure just seems high to me.

Kevin

Last edited by Kevin45; 08-06-2004 at 06:56 AM.
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Old 08-06-2004, 02:24 PM
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when i use my hvlp it's somewhere around 40-45psi @ the gun.

JB
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Old 08-06-2004, 04:13 PM
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The pressure you see is the pressure on the up-stream side of the regulator on the spray gun. The pressure at the tip of a HVLP gun is dropped down to ~10psig. The high pressure is needed at the gun to transport the higher volume there. You can't get that much air down a 1/2" hose at low pressure but you can if it is compressed to a high pressure. Once it is there, the pressure can be dropped across the regulator and the air expands to High Volume Low Pressure at the tip. Only other alternative is put 3" hose from the compressor to the gun which you wouldn't like too much!!

Conversely, on a conventional spray gun the pressure at the handle is pretty much the pressure at the spray tip, i.e., ~45psig.
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Old 08-06-2004, 05:33 PM
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When I use a siphon gun I use 40-45 psi.

When I use a HVLP gun I use 65 lb out of the regulater and set the gun at around 10 lb.

Troy
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Old 08-06-2004, 06:38 PM
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I hope you don't use HVLP much Troy, as Willys says, the HVLP gun will have 10 psi at the cap, you have to have much more than that at the gun. Depending on the gun it could be anywhere from 23 lbs (as I remember, maybe it is 29) like a SATA NR2000 to 50 lbs on a Sharpe Platinum.

The 10 PSI as I remember is what makes it an HVLP gun, low pressure= low overspray.

However, you can't check that without a special cap that has a guage. So setting the gun at the max inlet pressure for the specific gun will theoretically give you the 10lbs at the cap for maximum atomization. It isn't always true being I have one of the caps and had one for the Platinum when I was repping. The Platinum needed 55 lbs to get the 10 at the cap. That extra five pounds increased atomization quite a bit.


Kevin, what is "at the regulator"? Are you talking about a regulator on the wall or at the compresser? Or you are you talking about a regulator at the gun, that is two TOTALLY different things. In fact the length of hose, size of hose, what filters you may have and many other factors will GREATLY change the pressure at the gun if you are checking at the wall. In many cases you need the 80 at the wall to even get the 50 or whatever you gun needs.
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Old 08-06-2004, 07:11 PM
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I was talking 80 at the wall. But without a gauge on the gun how do you determine what is at the gun or just go at it by feel?
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Old 08-06-2004, 07:27 PM
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Martin, why do you hope I don't use a HVLP very much?

Kevin, The HVLP guns have a gage and regulator at the gun
That sets the cup pressure, that is where you set the 10 lb.
But you will have line pressure to there, I use 65 lb. to there.
It takes awhile to get used to them, as they are a little different.
The first few times I used one I forgot to release the cup pressure, sure makes a mess. Since then I've used up a couple doz of them.

Troy
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Old 08-06-2004, 07:39 PM
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Some painters have a good "feel" and just look at what the gun is doing. Plain and simple, it is like a guy who plays guitar up on stage and will tune his guitar during a song, just by ear! That always blows me away. Others you will see with an electronic tuner like I use. Some guys are very good at what they do and they say to heck with the gobbily goop and just have a "cheater" valve at the gun to change the actual pressure at the gun.

Personally, I have used a gauge for many years because it is a law here to not exceed the maximum air pressure on the HVLP gun, so I have just left it there. You could use a guage with quick couplings to check the pressure and then remove it to use the gun, some guys do that.

I just like the "warm goowy" feeling of seeing the pressure reading.

But honestly, unless it has been checked at the gun, that wall mounted regulator reading means NOTHING . You can loose a LOT of pressure and even more so Volume from the wall to the gun depending on what hoses, couplings, filters and what not you are running.

He may be doing it by ear and if he does and the gun does what it was designed to do, more power to him.

Quote:
Originally posted by troy-curt
Martin, why do you hope I don't use a HVLP very much?

Kevin, The HVLP guns have a gage and regulator at the gun
That sets the cup pressure, that is where you set the 10 lb.
But you will have line pressure to there, I use 65 lb. to there.
It takes awhile to get used to them, as they are a little different.
The first few times I used one I forgot to release the cup pressure, sure makes a mess. Since then I've used up a couple doz of them.

Troy
I am sorry Troy, I didn't know you were using a pressure cup HVLP. If you are setting the cup pressure at 10lbs (I don't know if yours should be there but you would know) with an inlet pressure of whatever the manufacture says, that would be fine.

I was thinking you were using a gravity feed, I jumped to the conclusion because the pressure cups aren't used much and I didn't even think of it. I just see so many people mistakenly think that the 10lbs the darn instructions state is at the gun when really it is at the cap. I have no idea why the gun manufactures have to confuse people with that crap when the darn gun doesn't even have the capibilities to check the darn cap pressure.

Sorry for the mistake.

Last edited by MARTINSR; 08-06-2004 at 07:39 PM.
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