How many gallons for a compressor?

As far as tank size goes, the main factor is how long you're going to be painting in a single session.

Larger tank means longer time between compressor "on" cycles, which means less total wear on the motor over time. A one gallon tank will have the motor turning on every few minutes. If you're only painting in 20-minute sessions, then that's ok. If you're going to be painting for several hours at a go, then those frequent on-off cycles will shorten the motor's life span, and you'll want a larger tank to spread things out.

A larger tank means the motor has to stay on longer to refill it, so as you go up in tank size you also want to go up in CFM. In order to limit heat build up you don't want the motor to stay on for a long time, as the worst wear happens when the motor is hot.

Base CFM to run just an airbrush isn't high (1 to 2 CFM, depending on the brush, but mostly in the 1 to 1.5 range), but you need to add to that according to your tank size. At the very least, you want enough overage to continue spraying while the tank is refilling. 2 CFM for a 1 gallon compressor is kinda the floor for what you want, IMO.

You can always get accessory tanks, so IMO prioritize CFM over tankage when pricing compressors. It's better to have a small tank that fills fast, than a large tank that fills slow.

When it comes to noise, be aware that most compressors that aren't marketed specifically for airbrushes wil define "quiet" or "silent" by construction site standards, not household appliance standards. I.e. "silent" doesn't mean silent, it just means "you won't need to shout to be heard over the motor", and "quiet" means, "you probably don't need hearing protection."

Ignore "horsepower" numbers. This is not a real or helpful spec # no matter what your needs are. It's basically marketing's attempt to "simplify" the numbers for laypeople, that actually oversimplifies into uselessness.
 
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