I recently picked up an HVLP conversion gun from an estate sale, never used, but with no manual. I am going to spray a Minwax satin polyurethane for a kitchen table top. Any advice on air pressure settings etc…. to minimize overspray, orange peel or other problems would be greatly appreciated. I have used an turbine HVLP for years, but it died, and I got this instead, to reduce noise. I also liked the idea of holding the gun without the weight of the quart can of liquid, to manuever into smaller spaces or spray from below. (The gun connects via hoses to a pot.) Thanks very much.
Jay
Replies
Most conversion guns require about 20- 25 lbs pressure at the inlet end when the gun is triggered, ie, where the hose connects into the handle of the gun. The only sure way to know if you've got this is to put a pressure gauge on the end of the handle of the gun and connect the hose to the gun through it.
You can roughly calculate the pressure at the inlet side by allowing so many pounds pressure drop per ten feet of hose. For 1/4" internal diameter hose the pressure drop is, if my dodgy memory serves me correctly, 5 lbs pressure loss per 10 feet hose length. This pressure drop information is out there on the internet, so don't rely on my dodgy memory.
With pressure loss through the works of the gun you're looking for about 7 lbs pressure at the air cap when the gun is triggered. You can get gauges to test this, but if you've got the inlet pressure about right the outlet pressure shouldn't be far out either.
All the other controls, ie, the fan width and fluid volume are the same as conventional guns. Slainte.
Richard Jones Furniture
This gun has a pressure guage at the top of the pot and a knob at the top of the pot to adjust that pressure, which I guess is supposed to reflect the pressure at the nozzle. I adjusted it to about 7.5 when the compressor was on 25. There are about 4 feet of hose from the pot to the gun. There are two knobs on the gun, one is for air flow to the nozzle and the other I think, is for fluid volume. I don't see one for fan width, unless I'm missing it. Where would that one usually be?
I tried it on the bottom of the table first, and it seemed to work pretty well with the pressure on the guage at 7.5. The small knob on the top of the gun varies the air flow and atomization, but doesnt change the reading at the pot. I just am not sure what the larger knob, below it on the gun does. I couldn't tell much perceptible change in the flow or pattern as I turned it while spraying.
Advice?
Jay
I'd guess you need about 30 lbs air pressure entering the pot, and with the loss of pressure to the entry point at the gun you wouldn't be far out. Fluid pressure leaving the pot would be lower at about 10 -12 lbs.
The knurled adjuster at the top to the rear of the gun adjusts the air volume to the air cap and this adjusts the fan width. Sometimes this adjustment is on the side of the gun but it doesn't sound like that's the case here.
The lower knurled adjuster, again at the back of the gun controls the fluid volume. To choke the fluid volume down turn it clockwise as seen from the back as far as it will go. This will stop any fluid getting up to the air cap. Raise the volume of fluid drawn from the pot from there by turning the knob anticlockwise a bit at a time.
Sometimes there's a third adjuster at the bottom the of the handle but as you don't have this valve it's not relevant.
I'm guessing a bit regards settings. You might try the gun manufacturer's or pot manufacturer's website for more information. http://www.devilbiss.com/if_main.asp for example carry useful information at their website. Slainte.Richard Jones Furniture
It sounds like what you've got is a single-regulated pressure pot system. For a relatively minor investment of $30 - $50 you could add a second regulator (& guage) and turn it into a much more versatile double-regulated system.
The difference between the two is that, with a single-regulated system, one regulator adjusts air pressure to the pot and also regulates air pressure to the gun because raising or lowering pressure to the pot also raises or lowers the available air to the gun. Adding another regulator allows you to adjust the pot pressure and gun pressure independently of each other. For example, if you wanted to drop the pot pressure (which would reduce the fluid pressure to the gun) but wanted to keep the atomizing air pressure to the gun pretty high then this would be impossible with a single-regulated system such as you have right now.
There are two primary ways of getting around this for less money, but both are much less precise and neither will get around the fact that reducing air pressure to the pot will also reduce air pressure to the gun. The most that these work-arounds will do is enable you to lower the air pressure to the gun without lowering the pot pressure. One would be to add what's called a cheater valve which would go at the base of the gun handle. If there is no tapped hole with a threaded plug already there then this is not an option. But the other type of cheater valve can still be installed. It is just a small valve with threaded fittings at both ends which you install between the end of your air hose (coming from the pressure pot) and the handle end of the gun. They cost about $10. Either will work... after a fashion. But doing it the proper way with a second regulator is the best and most useful way to adjust fluid and air pressure independently of each other.
Thanks. If I wanted to add this second regulator, exactly where would I attach it? Also, can you be any more specific regarding the type and size of regulator, and perhaps suggest a supplier?
Also, and this may show my ignorance, but if I am waiting overnight to apply another coat (either due to convenience or the need for drying), does the gun have to be drained and cleaned or can it sit over night? It would seem to me that since the fluid in the pot and the interior of the tubing is a closed system, not exposed to air, that I shouldn't have to clean it until I am done with the 3 coats I had planned ot apply. Thanks for the info.
Jay
It's my pleasure, Jay. And I would add that your ignorance of conversion systems is probably matched by my ignorance of turbine systems. It all comes down to what one is used to working with.
The way that my pressure pot is set up, both of the regulators have four tapped ports. As you are facing the regulator, the port on the left is where incoming air is introduced, the one on the top is where the pressure guage threads in, the one on the bottom supplies regulated air to the pressure pot and the one on the right has a short (2") section of threaded pipe connecting from the first regulator to the second regulator. The second regulator has a pressure guage on top, the bottom port is plugged and the airhose going to the gun is connected to the right-hand port via a threaded fitting.
Whether to drain the gun for an overnight is partly a matter of what kind of material is in the pot and partly a matter of personal preference. Catalyzed and water-borne finishes are best removed and the fluid line (and gun) flushed at the end of each day or the potlife of the material, whichever comes first. With non-catalyzed solvent-borne finishes I personally just leave them in the pot. But I do drain the majority of the material back into the pressure pot. Both the solids and any flatting agent present in the finish/paint will settle out and need to be stirred back into solution periodically. I don't like running the risk of flatting agent and/or solids collecting in the fluid line and thus potentially altering whatever goes through them the next morning, and that's my reasoning for not just leaving it in the line. The material in the pressure pot is easily restirred the next moring and then I'm ready to go again.
I'm fond of using an air nozzle (y'know... for blowing stuff off) with a rubber tip on it to blow the finish material back through the fluid line and into the pot. What I do is disconnect the air line to the pot and release the pressure inside the pot. Then I pull the gun trigger back with one hand, mate up the orafice at the end of the air nozzle with the aircap orafice on the gun and engage the air nozzle... thus phycially forcing the fluid inside the gun and fluid hose to be blown back into the pot. I don't do this with really high air pressure, though, because it can cause finish material to blow back into the regulator and that will ruin the regulator (which is easily rebuilt with the appropriate parts). But at that point I figure that enough of the material is out of the line and gun so as to not cause me any problems.
One last suggestion... if you intend to leave finish material in the pressure pot. I would install a simple lever-type cut off valve between the air coupler connecter on the first regulator and the regulator itself, and then close the valve for overnight storage. This will prevent air from getting in and causing anything to dry out or skin over. The needle valve on the spray gun will keep that end closed.
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