Feeding your HVLP gun

Howdy folks,

I am preparing to Awlgrip a freshly-restored boat, and have several questions about equipment. For years, I have been using a siphon-fed (cup hangs from the gun) HVLP gun with a 1.9 mm tip for repainting machinery and commercial equipment. I mostly work with high-solids alkyd-based enamels and primers. That gun is rated 11 CFM @ 30 PSI, but I have found I often need to crank the pressure up to 40 - 60 PSI to get the higher viscosity paints to siphon from the cup, and running at this higher pressure often results in blow-back and poor flow of the paint.

Because I have been less than thrilled with that gun, I recently picked up a pair of DeVilbiss Starting Line guns. These are gravity-fed (cup on top) HVLP guns, also rated around 11 CFM @ 30 PSI. They come with nozzles: 1.3, 1.5, and 1.8 mm. I anticipate having less problems with high-solids and high-viscosity paints using these guns, but now that I think back through my setup, I wonder if I was too quick to give up on my siphon-fed gun.

The rig is driven from a 60 gallon compressor, rated around 13 CFM at 90 PSI, so that should be no problem. I run thru a 1/2" regulator into a 50' coil of copper tubing cooled by a fan (to condense any moisture) before running to a fairly large separator with 1/2" fittings. The output of the separator is a 3/8" coupling which feeds 100' of 3/8" air hose. I know the air hose could stand to be shorter, but that's probably not my biggest problem. The output end of the air hose is a 1/4" I/M coupling, and on the handle of the gun I have a second mini-separator (1/4" fittings) and regulator. I keep the main regulator on the compressor at 80 - 90 PSI, and regulate down to 30-40 PSI using the mini regulator on the gun.

I'm wondering if the 1/4" I/M fitting on the gun, or the mini separator (which looks fairly restrictive) could be causing the siphon problems I've had all along with this gun. When I pull the trigger, the pressure on the mini regulator (right at the gun) drops significantly. I don't remember the numbers, but I would guess it probably drops from 30 PSI static to 10 PSI with the trigger pulled. I need to pick up some more 3/8" fittings tomorrow, but what would be the target here?

Muddling my logic further, my new Devilbiss gravity-fed HVLP (11 CFM @ 30 PSI) guns came with a air-flow control valve to be mounted on the handle of the gun. This is a needle valve that controls air flow rate, not a pressure regulator like I have mounted on my old gun. After reading about guys trying to maximize air flow to their HVLP guns, I do wonder why DeVilbiss ships an air-flow control valve with their guns. Especially considering there's already an airflow control valve built right into the handle of the gun itself...

Thanks in advance for any enlightenment you can offer. I will reserve my Awlgrip-specific questions for another thread.

 

Update:

I rigged the gravity-feed gun with the 1.3mm tip with the flow control, a mini separator, and a 3/8" coupler. I also picked up a new 50' x 3/8" air hose, which is much lighter than my heavy-duty 100' Goodyear hoses. I ditched the secondary regulator I mentioned before, and set the main regulator down to 60 PSI. I then used the flow control on the handle of the gun to set the handle pressure to ~30 PSI with the trigger pulled. After filling the gun with product, I played with that flow control and found the gun needed at least 20 PSI to atomize the paint, and seemed to work nicely around 25 PSI at the handle, with this particular primer.

I don't think I'll go back to the two regulator setup, as I think it was causing a lot of the problems I was having with getting the siphon action to work on my siphon-fed HVLP rig.

 

1) maybe you understand but you have to measure the air flow on the gun gauge with the trigger open and the fan set to max(your normal setting)> measuring on shut is irrelevent>

2) yourcompressor is probably too small to drive gun on long hose. if it runs off 220v standard household fitting almost definitely. you'll always get a peak on the presure when you pull trigger and it will drop (and keep dropping if hose is too long/compressor tto small)

3) the smaller the tip the less air you will need to atomise. the trade off is application rate. Then again it gives you more control. We spray with 1.8mm tip(devilbiss flg gravity) for 545 and 1.0mm(SATA gravity) for topcoat.

go to devilbiss site. They do a great guide to spraying. I think it is called "The abc of spraying". It goes into all the details but in easily understandable manner.

Best of luck!

(just finished off 60m-metallic superstructure, black hull)