Automotive Paint Tools
"How big of a compressor do I need to paint a car?" This is a common question. Well it all depends on the paint gun you will use, and possibly any air tools that you may use to prep the car. Compressor outputs are rated in cubic feet per minute (CFM) at a given pressure in pounds per square inch (PSI). Air powered device air requirements are usually spec'ed the same way, in CFM at a given PSI. Most spray guns of any kind operate in the 10-50PSI range. Compressor outputs are almost always specified at 40, 90 and for bigger units, 175 PSI. You want to insure that the rated output of the compressor you use is at, or preferably above, the air volume requirement of the gun.
Well how big of a compressor? How many horsepower? How big of a tank?
Well, to understand the relative function and effect of the compressor components, think of a compressor as a water pump and a big barrel. The horsepower spec determines how much water the the pump can move, let's say in gallons per minute (analogous to CFM). The barrel determines how much water the thing can "store up". Now lets say your pump is rated at 10 gallons per minute, and you have a 100 gallon tank. It will take your pump 10 minutes to fill the barrel (this calculation has little relevance to the compressor example, as the water isn't actually being compressed and the pump output rate isn't affected by the amount of water in the barrel). The pump shuts off when the barrel is full. Now lets say that you have a "tool" that uses 5 gallons per minute, and you attach it to a spigot on the bottom of the barrel. Once you start using it, the water level in the barrel falls and the pump starts again. But since the water going into the barrel is going in at 10 gal/min, and the water going out (what your tool is using) is at 5 gal/min, the pump easily refills the barrel and shuts off when it is full.
If your tool uses exactly 10 gal/min, the pump will kick in when the water level falls, but since the input and output are equal, the pump will run continuously while the tool is being used and the water level will not fall.
