HVLP Versus Standard Siphon Spray Guns

      Finishers explain the differences between HVLP and non-HVLP spray guns, and discuss their comparative advantages. November 13, 2005

Question
I build furniture in a one-man shop and would like to improve my finishing capabilities. In a previous life I used a off-the-shelf gravity-feed HVLP gun to shoot small amounts of automotive lacquers, catalyzed enamels and a bit of urethane. Now I'd like to get a gun appropriate for woodwork, and a gravity gun seems appropriate for the scale and budget of my operation. I see, however, very similar looking guns, of which some are marked HVLP and some are not. Can anyone help me parse the difference between the two, and also educate me a bit on appropriate tip sizes?

Forum Responses
(Finishing Forum)
From contributor L:
HVLP vs standard gun - the HVLP uses more air but is more efficient at laying more paint on the surface. You get less blowback and less overspray with HVLP. The standard siphon gun will give you a better atomization on average at the expense of efficiency of coverage. More expensive HVLP (DeVilbis, etc.) can give a good droplet size (small). I almost always pick an HVLP over a siphon if I need great atomization. If overspray isnt an issue I might use a standard high pressure gun.



From contributor F:
I just dumped my gravity conversion HVLP from PC for a siphon model with turbine from Fuji. What a difference. Believe it or not I just sprayed a couple doors and 4 windows and the finish was fabulous. The overspray was so little I got literally not a drop on me, I got more on my hands relieving the pressure in the can between coats than from spraying.


From contributor J:
I use two guns - both are HVLP. The larger is a Star HVLP and the other is a mini AIRCO (both are copies of SATA). I find both produce excellent finish quality with Campbell precat lacquers as well as automotive paints. They are both very sensitive to tip sizes versus viscosity (thinning is very important I find) and air pressure to minimize droplet size versus overspray.

I have 2 air dryers, the first is the standard filter with glass collecting jar after about 30 feet of hose then the final Snap on Dessicant dryer and dessicant air dryer/pressure regulating valve. I use a 3/8 id hose and large couplers so there is lots of air flow and not too much pressure drop at the tip of gun. Usually I use a 1.7mm tip on the Star and a 1.0 or 1.1 on the Mini.



From contributor M:
HVLP - high volume, low pressure guns generally use less air than conventional guns. They also have a slower transfer rate - you put less material on in a given time, but they have better transfer efficiencies (less overspray) than conventional guns. Conventional guns put out more material and need higher pressure to atomize the fluid. They also have more overspray.

The LVLP may be similar to the RP (reduced pressure) type guns that have higher output than HVLP. They have better transfer efficiencies than conventional guns, but not as well as HVLP. It is for someone who wants better output than HVLP but not as much waste as conventional.

The guns that I have been talking about are conversion type, not turbine. They can be connected to a regular compressor. There are also some guns that are designed to be used with smaller, portable compressors. Some can even be run off of a 2 hp compressor. I would get the specs from your material supplier and visit someone who sells equipment. They should be able to get you set up.



From contributor C:
I would recommend checking the consumption on the gun. Some HVLP guns use as little as 4 cfm, and some as much as 12 cfm. The same holds true for siphon guns. It depends on the model, you can't really compare the small one of one to the large one of the other. Its like comparing apples to oranges. Most of the better HVLP guns use a lot of air, 10 and 12 cfm or more. That's why most manufacturers recommend bigger hoses with their HVLP's.


From contributor G:
HVLP guns will actually use up to 25 cfm on some guns, through design nothing else. What that means is your Home Depot compressor can actually starve the gun due to volume loss, especially if you are using quick disconnects that aren't HVLP rated.

The upside of HVLP is that it can get upwards of 50% transfer efficiency on certain materials, etc. The Siphon gun at best will be 20-25%, usually 10-15% transfer efficient. HVLP has slightly higher TE than conventional (guns not marked HVLP). HVLP is also OSHA approved and up-to-code EPA wise, as long as it is used along local parameters. HVLP should never be used with less than 3/8 air line, as it could starve the gun.



From contributor F:
This is exactly the reason why I bailed on the conversion gun and went with a turbine system. My PC gun, although well built, needed 9 cfm and my compressor wasn't ever going to deliver it, and with a 1/4" fitting I doubted I was ever going to get the HV part of it. Rather than buy a bigger compressor (which I had no other need for), I went with a turbine.


From the original questioner:
I'm definitely looking at conversion guns rather than turbines, but I've got to aim for one that will be functional with my fairly small compressor. I've got an ancient 2 hp cast-iron, belt-driven model, which I realize is far from ideal.

So here's a follow up question: exactly what difference does it make if your compressor can't quite keep up with your gun? Is there necessarily any harm in pausing for 20 or 30 seconds once in a while to let it catch up? Are only certain finishes tolerant of this? I ask because many of you are in production environments where speed seems to be everything. I do only custom, one-off work, finishing consumes relatively little of my time, and a spray gun that slows me down by 20% won't make a bit of real difference to me; finish quality and ease of use is really what I'm concerned with.

In a couple of other threads I read a lot of positive comments about the Astro guns. Supposedly the HVLP model I was looking at uses 10 cfm at 41-44 psi, and the LVLP gun uses 7.8 cfm at 40 psi. Does anyone have any thoughts?



From contributor M:
To the original questioner: You are a one man shop, right? I didn't think that you were asking for feedback on 25 cfm guns. There are exceptions, and I was speaking in general terms.

You can get by with a smaller compressor and a larger gun if you are spraying smaller pieces. But if you are working with a larger piece, either panels or a larger cabinet (ecenter) then you may run out of steam. Especially for solvent based material, you want to spray with overlapping coats and maintain a wet edge. If you stop for a minute to let the compressor catch up, your product may begin to flash and you will have overspray problems. When I shot water-based products years ago, with gravity conversion gun and a small compressor, it was less of an issue. But I am sure that things have changed in the past 10 years with water-based product.

Check the output of your compressor and have this information ready when you talk to the salesman. You also need to let him know what you will be shooting. Try to find the viscosity of this, and he can help you with needle, nozzle, and aircap sizes.



From contributor F:
Here is exactly what happens when your compressor does not produce enough cfm to keep up with the gun: the compressor motor runs a lot more, and with a larger piece you may exceed the duty cycle of the motor. For example, the common PC 6 gallon pancake compressor most often used with nail guns puts out I think 3 cfm at 45 psi. It also has a duty cycle of 50%, meaning in one hour the motor should not run more than half the time. The PC HVLP conversion gun requires 9 cfm. While spraying the tank quickly runs low and then the compressor has to run continuously to keep the pressure up, and even then it is losing ground - soon the pressure at the gun is below the 45 psi it usually wants. Then you have to stop until the tank recharges. If you do this too much you burn out your compressor motor.

Needless to say, this can be frustrating, and I spent a lot of time letting the motor cool off. Not bad if you have something else you can do while you wait, but it made it hard to get a large piece done. Perhaps I could have pushed the motor harder, but I didn't want to risk it. In the end I found spending the money to get a turbine unit a worthwhile tradeoff.



From contributor J:
When I started using HVLP I rewired an old Sears 10 gallon compressor (high speed pump not old style) to 220. For big jobs I piggy-backed with another compressor of the same size that was normally used for nail guns. It was way less expensive than buying a big high capacity compressor. A house fan directed at compressor cooling fins really helps to keep the temperature down as well.


From contributor C:
One of the biggest problems with a compressor that's not big enough is the heat. After it runs for a while it will be next to impossible to keep the water out of it. The air will be hot. I have the Astro gun you speak of and yes it's true it pulls a lot of air at 44 psi but I find it performs better
at 30 psi or less and at that pressure it uses a lot less air, maybe half as much.

It's my favorite gun. And you can buy a bag adaptor kit by DeVilbiss for it which will allow you to seal a zip-lock bag in the cup that works really nice. It makes clean-ups a breeze and you
can spray upside down with it.



From the original questioner:
I have ordered one of the Astro guns, so we'll see how it goes fairly soon. Is there a specific part number you can provide for the appropriate DeVilbiss bag adaptor, or is it a fairly generic item that I should be able to find easily?


From contributor C:
If you go to Spraygunworld on the internet and search their site by keying in "liner kit" you'll see them. And they are only about $20. Sears sells 10 of the same bags alone for $10 so the kit is a bargain.


From the original questioner:
I really like the Astro conversion (1.7 tip) gun. My 2 hp compressor is usually set to top out around 110 psi. With the diaphram regulator set to the 41-44 psi the gun is rated for, I can spray wide-open for about 30 seconds before the compressor kicks on. This compressor will maintain tank pressure at 95 psi indefinitely, with the gun wide open. The compressor will build the pressure back up to full and shut off about 30 seconds after I stop spraying. If I were regularly doing whole kitchens this setup would be woefully underpowered, but for smaller projects its fine.



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