A source of reliable, clean air for your spray guns is one of the main elements needed to get the professional finishing results we all want.
In this post, I’ll focus on air compressors (we’ll cover turbines in future posts) and show you the factors I consider most important when selecting one.
So let’s jump right in…
Key Factor #1: Air Volume Requirement. Selecting an air compressor starts with quantifying your total air volume requirement (measured in Cubic Feet per Minute or CFM, at 90 psi).
I have one air compressor dedicated to my spray booth (a separate one serves the rest of my shop), so that’s the example I’ll be using here.
Inside my booth are two air lines: one for spray guns, and another for my small air blow gun.
To perform effectively, the “thirstiest” of my four compressed-air guns require 15.2 CFM at 26 psi. And because I am using only one gun at a time (the air blow gun is only on for such a brief period, I’ll disregard its needs), my spray booth’s theoretical CFM demand is 15.2.
With a generous 20% overage factored in, the actual demand is 18.24 CFM, so my air compressor’s pump needs to be able to provide that volume of air.
Key Factor #2: Motor HP Rating & Duty Cycle. Some manufacturers exaggerate the horsepower ratings of their electric motors to attract buyers. But by using the rule of thumb that 1 HP can generate roughly 3.5 CFM at 90psi, you should be able to determine the approximate real HP requirements of any air compressor.
Another part of the discussion on motors is its duty cycle. A 5 HP compressor with a 100% duty cycle produces more continuous air than a heavy-duty 7.5 HP compressor with a typical 75% duty cycle. More is better!
Other Factors: Tank size, Electrical Requirements, Operating Noise. Let’s look at these one by one…
- Air Tank size/shape. A 60 US gallon tank is a good size for most one-operator spray booths. If your shop’s demands are beyond that, your supplier should be able to help you select one that best meets your needs. In a similar manner, if the only location for your unit is in a low ceiling mezzanine or other restrictive space, remember that air tanks also come in horizontal configurations. And if you’re planning to locate your compressor in an out-of-sight location, consider installing an auto drain system to help ward off tank rust-out.
- Electrical Requirements & Connection. Before selecting an air compressor, consult with your electrician. They should know the local codes and your unique situation and can therefore determine if your shop’s existing power panel is able to handle the load, as well as to give you an estimate on any electrical installation work.
- Operating Noise. Air compressors are noisy machines, some more than others. Installing sound-deadening insulation in the room where your compressor will be located will mitigate this factor, however there are cases where this may not be practical. Many manufacturers provide the decibel (db) ratings of their machines, so if noise is a factor, this information should help in your selection.
A well engineered and manufactured air compressor that is sized uniquely for your compressor-based spray guns can make all the difference in the quality of your finishing products. If you’re about to take the plunge and purchase a new air compressor, I hope this information will prove helpful.
Any comments or questions about this article…or requests for future topics? Please share your thoughts or see what others are saying in the comments section below…and I’ll respond to as many as I can. I’m here for you!
– Marty Schlosser
The information here is somewhat misleading. It does not address scfm and air compressibility. It would lead the layman to buy a larger compressor than necessary. Using your example of demand of 18.24 cfm @ 26 psi, the compressor demand at 90 psi is 40.7/104.7 or 38.8% of gun cfm, which is 7.1 cfm @ 90 psi. While you do mention duty cycle of the compressor you do not mention duty cycle of the gun. The hobbyist does not have to have to the same gun duty cycle capability as the production sprayer – waiting a minute to let the air compressor “catch up” is not a problem for the home enthusiast, who can do away with the 20% factor, reducing the requirement to 5.7 cfm @ 90 psi, which is a much more affordable compressor.
Douglas – Thank you for your post and for your patience in waiting on a reply. We are working out a kink in our blog admin page, and I want to share Marty’s reply with you and our readers. Again, thank you.
-JW-
Thank you for raising these issues, as it will allow me to delve into them without concern for increasing the text size of my blog.
The 7.1 CFM @ 90 PSI figure which you calculated represents the requirement at the gun, and not at the compressor outlet. The various fittings, connecting hoses and filters along that path leading from the pump to the air gun all take their toll, which is not inconsequential. Then of course there are the realities of air line leakage and equipment performance degradation over time. And for those reasons, 4 out of the 5 suppliers I spoke with in preparing this article, cited CFM figures akin to mine. The other one cited a somewhat higher figure, explaining that an overworked air supply system will not only perform poorly, but due to overheating, will fail much earlier in comparison to a more realistically sized system.
Regarding your gun duty cycle issue, whenever any operator – hobbyist or production finisher – is required to wait for their air compressor to catch up, the wet edge necessary for a unified finish will be lost. It is not a simple matter of waiting and carrying on. In the best case, it will be a matter of starting a new wet edge. More than likely, that panel will have to be reworked in one fashion or another. And in both cases, it’s an issue of finish quality and time, which are valuable to both the production finisher and hobbyist alike, albeit for different reasons.
As an alternative to a less-than-optimal air compressor to save money, a turbine-based HVLP system may be a better fit for many. I hope you’ll catch my future blog where I’ll be exploring the world of turbines.
– Marty Schlosser –
Hello,
I make guitars, ukuleles, cigar box guitars and other small woodworking projects in my garage that has limited space. I have a Husky 60 gallon 175 PSI 1.7 HP oil free compressor with a 20 gallon external tank. This compressor is rated at 5.1 SCFM at 90 PSI, 6.8 SCFM at 40 PSI. I am just getting started but I purchased a spray gun from a luthier named Jeff Jewitt whom is well known in the woodworking and finishing community (having authored books and taught classes on the subject). It is pretty hard for a guy like me to tell what will fulfill my needs in this area. I found your luthier water based lacquer and bought a quart of it last year. Will this setup work for me to lay down several coats on acoustic and electric guitars and small instruments? I guess there’s nothing like actually doing it, which I should do using some scrap wood in a simulated environment while timing the process, right?
What is more important to a setup like mine? Capacity or SCFM? I know there are several questions in there, but any help or advice would be welcome.
One last thing: What is the unopened shelf-life of EM-6000 water based instrument lacquer? I have had this quart and stored it in a stable temperature for about a year. Thanks, Steve
Hello, Steve, and thanks for writing in. It sounds as though you’re really into the luthier world! Knowing Jeff’s sterling reputation, I’m sure he discussed various spray gun options with you and took into consideration your compressor’s characteristics before recommending which model of spray gun to purchase. Assuming that to be the case, I think you’ll be just fine. Your idea to undertake several tests using scrap wood to see how things work out for you is the way to go. As an aside, I ran an advanced workshop for Fuji Spray a few years back where we provided the students pieces of thin lauan mahogany shaped like guitar faces for them to play around with (shading and sunburst effects, etc.), so if you could do something similar to: 1) see how well your spray gun & compressor combination works, and 2) to practice your spraying technique.
If you notice that the compressor kicks in while spraying, finish off the pass you’re doing at that time and wait for your compressor to catch up. Conversely, if your compressor is able to keep up with you well enough to allow you to finish spraying that coat, then go ahead and continue that coat. (You can tell by looking at the mini-compressor gauge at the base of your gun; if the pressure shown while the compressor is running is the same as before, then your pump is keeping up with the eamand). Whether or not it’s up to the task, I’d still recommend you wait until the compressor has fully caught up before proceeding.
As regards the shelf-life of EM6000 lacquer, I’ve had that product perform as well as new, even as long as 24 months after first opening it. So as long as it’s never been frozen and the lid was properly sealed all that time, it should be just fine. Again, confirm this with test pieces before risking trying it on the genuine thing.
Hope this helps and please let us know how things work out for you.
I have two compressors that I used with conversion guns… a 13 gallon Home Depot compressor; since it runs on 120 volts household it cannot be over 1.5 real hp. I found it was more than adequate for high volume spraying – it takes me 20-30 minutes before I catch up with the compressor and air tank, and after 20-30 minutes as a semi-pro furniture makes, I needed a breather (and to refill the gun) anyway. The compressor caught up faster than my body did.
My second compressor is a tiny unit that I bought rather than an air tank. I have used it for touch up spraying and occasional finishing nails or pins (attaching trim).
A friend who has a furniture restoration business is very happy with his hot dog compressor for spraying
I now do 99% of my spraying with a turbine.
I have an artist friend who loves her 5 hp compressor with a large (60 gallon?) air tank that she uses for wood carving.
Charlie, thanks for writing in, as it’s always interesting to hear others experiences. I am assuming you’re using conversion HVLP guns from the comments you’ve made. I am pleasantly surprised to hear how well your compressor is working out for you. (By the way, I can relate to your comment about needing a breather from time to time, as I’m getting “up there” myself).
I took the time to wander over to your website and really enjoyed looking at your cabinetry. Being a furniture maker and designer myself, it’s nice to see what others are doing. Keep it up!
Charlie – Good to hear from you. I hope that you contribute more to our blog, as you offer a wealth of insight to the woodworking community.
-JW-
Marty, Thank you for your detailed reply. My confidence is lifted by that. I purchased one of Jeff’s kits (it is the one he sells when you don’t know what kind of spray gun to purchase). I’ll let you know how it turns out.
I have heard it is good to warm up the contents (in the spray gun’s content container) in a pool of warm water (77 degrees comes to mind) prior to spraying any kind of lacquer/finish. I was thinking about keeping a basin with an aquarium heater in it for the purpose. Is that overkill?
Thanks again for your reply.
Steve, under most circumstanses there’s no need to warm up the contents of the finish. Marty Schlosser