HVLP question

I purchased a cheap HVLP gun at HF to do some spraying. The question I have is about pressure. the manual states to run it about 40psi, and never run it higher than that. i have a small regulator at the gun, and one at the compressor end. when i set the compressor regulator at 45, i get about 40 at the gun (i have to use about 75' of 3/8" hose to get where i need to spray as the compressor is a large tank in the garashop). however, when i start spraying, it drops down to the 20 psi range whilst i'm spraying, and going back up when i release the trigger.
when i crank the pressure up at the compressor regulator, the pressure then, of course, goes up at the gun. when i start spraying, the pressure drops pretty quickly, then rises back up when stopping spraying.
this is my first experience with an hvlp, so is this normal? is the 20psi during spraying good enough? it seems to be spraying a pretty fine mist at 20psi when i did my testing with plain water.
what i'm spraying is mostly water anyway, so it shouldn't be too much different viscosity.
thanks, charlie
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On Wed, 16 Nov 2011 11:00:20 -0700, chaniarts wrote:

Use a much larger hose for the first 60 feet. basilisk
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Keep the pressure up at the compressor, just about at its limit. You are going about it correctly by controlling the output to the gun with a separate regulator at the gun handle. More compression behind the gun will assure you that your compressor will not cycle as often as low pressure.
I have a couple of those HF HVLP guns and they are surprisingly good. I don't know your model (not that it would matter) but I usually find the sweet spot for spraying at about 12 - 18 pounds at the gun. If you are shooting at 40psi to the gun, no doubt you are draining your compressor quickly. Remember, High Velocity, LOW Pressure.
If you can't get your material out at lower pressures, thin it out more. When I was learning to shoot with a HVLP gun powered by a compressor, I set it up like a high pressure gun. My compressor couldn't keep up, I had lots of overspray and drift, and didn't see any savings in material. After really woodshedding that gun and getting some excellent advice, I got it to where I want it.
Remember, and HVLP setup will not atomize your finish material the way an old fashioned high pressure spray will. It isn't designed to work that way, hence, it simply won't.

Shooting water shouldn't be a trick for any gun/compressor at any pressure. Keep the pressure low, and adjust your feed and pressure as needed to keep the water flowing.
Robert
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On 11/16/2011 12:36 PM, snipped-for-privacy@aol.com wrote:

it's usually used for sandblasting, so is a large tank and relatively large pump. i ran a test for about 15 minutes with plain water, and it never kicked on to refill. the tank fills to 125psi but the regulator there is normally set to 75 for blasting.

this is the one i picked up <http://www.harborfreight.com/professional-automotive-hvlp-spray-gun-kit-94572.html>

it's probably 98% water, so i doubt that it can be thinned more. the product is the liquid that is in Scotchguard rattle cans purchased in bulk. i'm spraying a lot of furniture.

thanks for the help all.
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