This depends on the spray gun you have, or intend to purchase. You typically need to have a compressor that can provide the spray gun with its required CFM. Low CFM spray guns only require around 4-6 CFM to operate effectively while quality HVLP (high volume low pressure) spray guns will require upwards of 20 CFM of pressurized air. Some conventional and common spray guns will require around 10-15 CFM. The conventional guns offer a far improved quality finish as they break up the paint better than the lower CFM spray guns.