I started with a De Vilbiss 4-hp, 20 gallon tank. Five seconds of die grinder or air hammer, five minutes to regain pressure. Do you have any idea how long it takes to chisel out ball joint rivets that way? Maybe eight seconds of impact gun, but you can hear it losing speed towards the end.
I now have a Sandborn 6.5 hp, 60 gallon tank. It will not keep up with a rubber-tip blow gun, until the pressure drops to about 50 psi. Better with die grinder, very acceptable with an impact gun. You wouldn't want to even consider using it with a sandblaster.
If you're gonna actually USE an automotive air tool, don't waste time and money with less than 6 hp-60 gallon tank. Go for an 80 gallon tank if you can, and if you're bucks up enough to assure proper electrical wiring, head straight for the two-stage, 175psi units. You can regulate down, but it is impossible to regulate UP. Set your regulator so that with 175 in the tank, you have 90 psi AT THE INLET OF YOUR AIR TOOL, WHILE IT IS RUNNING! You will NOT get rated power of your impact gun or air ratchet otherwise. This is why most air tools seem gutless- not enough pressure. (you will have to temporarily locate the pressure gauge to the tool inlet port.) With a typical single stage compressor, don't bother to install a regulator, you need all the air you can get. 120 psi in the tank gives you about 80 at the inlet of your tool, while operating. (depending on the tool and air hose, of course. )Sure, you may have 120 at the tool before you pull the trigger.
Personal opinion: Stay away from Sears or any other "oil free" compressor, unless you plan to spraypaint. Durability is found in an ALUMINUM compressor that has iron sleeves, and a supply of dead, processed dinosaur in the sump. Change the dinosaur now and again, and it'll work fine forever.
[This message has been edited by Schurkey (edited 11-16-99).]