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How do I know what size air compressor is needed for my spraygun or air tools?

Small repairs can be tackled using aerosol spray cans and hand sanding, but more ambitious projects will probably have you thinking about buying some equipment - air compressor, spray gun, amd maybe some air tools.
The most expensive piece of equipment will almost certainly be the compressor. Air compressors are advertised and rated by their air displacement - quoted in cfm, cubic feet per minute, and maximum working pressure in PSI (pounds per square inch) or BAR. Whilst this is a good comparitive guide, it does not give the complete picture.

Most cheaper compressors will be of the piston-type pump, mounted on a steel air receiver for air storage, though some portable units may well use a small turbine unit and a 'constant bleed' gun with no air storage.

The advertised CFM, e.g. 14cfm, is the piston displacement. That simply means the volume of air the compressor pump moves per minute AT NORMAL ATMOSPHERIC PRESSURE. The important figure is often hidden away in the specifications, or not quoted at all, which is how much air is actually supplied at a specified working pressure, such as 70psi.

This amount of air delivered at a working pressure will always be far less than the displacement figure - that's just down to simple physics, not a deliberate attempt to fool anyone. This can vary depending on the pump efficiency (or type of pump) and only the manufacturer will be able to provide the actual figures. Fortunately a good general rule of thumb is to use the motor size as a guide - motor horsepower multiplied by three will give a good estimate of the air supply available at a constant working pressure of about 80psi.

Air Compressor advertised at 14cfm, fitted with a 3hp motor
3 x 3 = 9cfm constant supply at @80psi

Therefore, a compressor of this size would be able to run a spraygun or airtool with a 9cfm setup continuously without running out of air (i.e. pressure dropping below the working pressure required).

Of course other factors can affect the air supply available, primarily the size of the air receiver (storage tank) and the length of pipework/hose between the compressor and the air tools.

A small air compressor pump may well be fitted or connected to a very large air receiver. This will mean once the receiver is filled to capacity at say 150psi it will take longer to use the stored air before the pump itself is required to 'top up' the tank.
The downside to this arrangement is of course that the air receiver will take much longer to fill in th first place, and longer to recover once the air supply is depleted.

The longer the length of hose/pipework, the more the working pressure at the end will reduce. This can be mitigated to some degree by using larger bore pipework.

Further information about the tools you could use if you purchased an air compressor can be found on this page.