Here's something I found on spraygunworld.com
SPRAYGUNWORLD.COM
CFM QUESTIONS
If you have a small compressor (smaller than 5-6 HP and a 30 gallon tank - less than 11cfm), we have a few recommendations:
Many professional HVLP guns require around 9-15 cfm. But Don't worry, many Serious Hobbyists and woodworkers have small compressors. Many guns are rated to run on a 20-30 gallon/5-6HP compressor which run around $260 at Sears. We also have options for you if you only have a 1HP or 2HP Compressors or compressors rated below 6cfm@40psi.
How BIG a compressor do you need?
Most air tool manufacturers will tell you how much air each tool needs and at what pressure it needs it. The quantity of required air is usually expressed in units called "CFM", "SCFM", or "ASCFM" (Cubic Feet per Minute). Next, look at the compressor's specs. Most compressors give you two numbers the "Displacement CFM" which is the the raw pump power and also something called "free air @ some psi" or "delivered air" or "SCFM @ psi" which is the number we are interested in. It should look something like this "7.6 CFM @ 40 PSI". Paint guns usually operate at very low pressures but require a high volume of air at that lower pressure ("HVLP" means "High Volume, Low Pressure"). So if you're painting, check the compressor's actual air output ("free air" or "delivered air") @ 40 PSI.
*If you only have a rating @90 psi no problem you should be able to add between 1 & 2 cfm as you drop down to 40psi depending on how good your motor is. Generally name brands have better motors, but this is not always the case.
Can I just look at the PSI & HP rating on the compressor and know the CFM?
Not really, The PSI is the maximum pressure your tank can hold. This varies by manufacture. Since most guns use about 30-40psi, it is irrelavant if you tank can hold 100psi or 200psi as you will only need 30-40psi to spray. Also HP is not as relevant today, because many consumer grade compressors are using what we call "exploding or RED line" HP ratings. The HP rating as your compressor red lines and explodes is not important to you. In other words, HP ratings today are very inflated numbers. This compares with older compressors, that have true HP ratings. What is a true HP rating? Well 1 HP should be able to produce about 3.5 CFM. A true 5 HP compressor should be able to produce about 15-18 CFM. Have you been fooled by your HP rating? Don't feel bad, most compressors today are not true HP ratings.
Does this mean I can't use a professional gun without a huge compressor!!!
No, No, No! It simply means that you can't run your spray gun continuously nonstop. Most of our guns rated at 7-10 CFM can be operated with satisfaction on a 2-5HP single stage compressor rated at 6.8 @ 40. Guns rated at 9-12CFM can be operated with satisfaction on a 5-6HP single stage compressor rated at 8.6 @ 40. If you have a compressor rated below 6.8@40, we have a special selection of professional low air guns and mini guns which operate on as little as 1HP.
How about a compressor rated at 8.6 @ 40 and a 20 gallon tank? Unless I were doing production work I don't feel you need an upgrade. You can spray an Astro Pneumatic Gun for example, rated at 10cfm at 40psi and it works just fine to spray four interior six panel doors one right after the other. By the time the compressor needs to catch up I need to give my arm a brake anyway. Or I need to refill the cup. Once you use a professional gun like the Astro Pneumatic, I'm sure you will like it as much as everyone else."
If my compressor is small are there any options for me?
Yes, Yes, Yes! Certainly! Compressors are simply tanks with motors on them to compress the air. You can always add anther spare tank to get more holding capacity that can be connected and disconnected when needed. You can also chain compressors together with a simple T fitting. Compressors are sold for as little as $38.00 for 4cfm@40 at your local hobby store and you can simply add such a compressor to your existing setup. (i.e. stick them together). This is another way to avoid a 220V outlet which many large compressor take. You can run an extension cord from another socket or fuse and chain these units together.
So why not buy a spray gun with a lower CFM rating to match my compressor?
Simply put, it is the air the breaks up your paint. Remember, professionals use about 10-15CFM to break up their paint and get the best atomization. We focus on professional guns to be used by the most particular professionals and serious hobbyists. An analogy: Low CFM guns on the market are like 4 cylinder cars. We focus on 8 cylinder professional models. For Low CFM Guns, you need to have superb engineering and precise manufacturing to get paint break up power in a "4 cylinder car" as compared to the breakup power in our "8 cylinder models", and that usually means you're paying more money for less performance or you are buying a very lousy gun with poor performance, orange peel and the like. Please, do not opt for an inexpensive low CFM guns, you will not get the fine atomization desired by a professional painter.
So am I out of luck?
No, we often encourage you to get a good size compressor, but at SprayGunWorld, we have searched far and wide for the finest low CFM guns on the market. We have created a low CFM gun selection for those who have small compressors. In our search, we have looked for high quality, mostly USA made options. Our offerings are limited to what we consider the finest low air guns on the market with - all with precision manufacturing. Please consider them "6 cylinder models".
If I am getting a compressor, what should I buy?
If you are purchasing a compressor you should always get the most CFM and the Highest PSI rating you can afford. But realistically, we recommend as a good starting point a 5-6HP compressor 8.6 CFM @ 40PSI. An example is the craftsman 6HP that runs for around $260 at Sears or a similar unit at Home Depot of 7.2 CFM@40PSI and the like.
Oil, Oiless?
The cylinders in a compressor generate a lot of heat. Some of this heat is caused by the compression of the air itself, but a lot more would be created if the piston caused too much friction against the walls of the cylinder on every stroke. There are two solutions currently used by compressor manufacturers: lubricate the pistons with oil (the way your car handles the same problem) - or line the cylinders with something very slippery (like Teflon). So-called "Oil-Less" compressors use solution number two. It's cheaper to manufacture a pump if you don't have to worry about circulating a lubricant (and giving the user a place to drain it and add more, etc.). So the primary advantage of the "Oil-Less" design is money. Advantage number two is you don't have to worry about checking the oil level - ever! But without oil to circulate and help draw away the heat, "Oil-Less" pumps will eventually get a lot hotter than lubricated pumps. Also the slippery lining will eventually wear away and you'll need to get the pump rebuilt. So the cheaper compressor will work fine for light-use applications. And if you don't use it a lot, it may well be all you'll ever need. But for day-in, day-out professional use, remember that there aren't many cars on the road with Teflon cylinders.
