The ability of using a certain type of gun/airbrush does not depend only on the HP of the compressor, it depends mainly on the pressure in the whole air system (tank and pipes) and the volume of the whole air system. The needed pressure for using a gun/airbrush is reachable by almost all air compressors. The problem is for how much time you can have that pressure in your air system while spraying. This time depends both on the HP of the compressor and the volume of the air system. Here on eGullet you mainly talk about the HP of the compressor, since small compressors can't keep up with a continuous use of a big gun. But you can solve this problem coupling a side tank to the compressor, such to raise significantly the total volume of your air system. There is a big difference between having a compressed air reserve of 20 liters and one of 100 liters. If you have a first system with 20 liters, a second system with 100 liters, then turn off both compressors, you will be able to use the gun for 5x time with the 100 liters if compared to the 20 liters. The problem with small compressors and small tanks is that you reach low pressure pretty soon and the compressor can't keep up. If you couple a big tank to a small compressor then you raise significantly the time of use. When spraying molds you are not using the compressed air for 30 minutes in a row (like what can happen in an industrial setting), you are using it for few minutes, then stop for some minutes, then resume, then stop. With this work flow a small compressor has the time to raise the tank pressure during your pauses.
So you just need to buy a big tank to couple with your small air compressor. Air tanks are not expensive, they are much cheaper than upgrading the compressor. Only problem is that it will take more time for the compressor to reach maximum pressure in the air system. You just need to turn it on when you reach your work place (before changing clothes and starting your work day).
Teo