I have 3/4 inch galv. piping from a 5 hp 80 gallon 17 cfm @ 125 psi compressor.
My parts house sold me two guns:
1. For primers--- API P/N91601, I got no instructions at all with this one.
2. DeVillbiss GTI 620G, HVLP, havent even opened it since Im not ready for it.
I'm going to practice with some primer and dont know what pressure to use, help!!!!

Primers are easy to practice with, your guns should come with a suggestion for airpressure so read any info in the box. Mix your primer as suggested and strain it, I'd start with 20lb.s of air pressure and see if the texture is ok at full trigger pull holding the gun tip 6-9 inches from the test panel surface. If the texture is bumpy then increase your air pressure to allow more atomization. If the primer is drying to fast (extremely fuzzy on the edges) because of improper temperature, your test results will not be right. What I strive for when spraying primers is to get the material on the panel at the desired texture with the least amount of pressure necessary to eliminate unwanted overspray.
Many of the shops in my area have gone to roll on primers which eliminate overspray and gun cleanup.
Spraying basecoats and clears are another story. I apply my first two coats of base fairly wet for coverage with no attention to metalic layout then straighten the metalics with two to three continuous light coats.
I like to apply the first coat of clear medium wet making perfect atomization and solvent selection critical, then I follow up with 1 or 2 more wet coats with 20-40 minutes flash time between coats depending on the cure rate on that particular day.
Get the primer application down first, pay attention to pattern overlap and trigger control to practice up for the paint application, Just remember the spray gun is like a carburetor-in respect to fluid and air mix. Air to fluid ratio is a constant, if you adjust the trigger travel to the minimum you will also have to make an air reduction to maintain the ratio. There is no perfect one setting for a spraygun in my opinion, when I spray jambs and edges for example I dial the volume down both air and fluid for more control on these smaller areas-why spray full blast/wide open on these smaller areas and waste all the material on overspray and increase the risk of runs etc.?
Fill your gun with water and play with the adjustments watching your pattern in the air and you'll easily see what I mean by proper ratio.
Hope I shed some light on this, I've had many people bring their guns to me in the past to have me adjust it for them before they paint :rolleyes: -impossible given the different viscosity of materials, application style etc.

Thanks,

Exactly the kind of info Im looking for. That cheap primer gun I bought had nothing but a parts list with it, nothing else.

After practicing I want to spray my frame with POR 15. I know I can brush it but spraying will be easier and I want time behind the gun.

Any thoughts?