Bob,
if you must use the propel, heres a tip; set the can of propel in a pan of hot tap water while you spray. This helps keep the pressure up inside the can.
As you use the propel, it starts to build-up frost around the top of the can, this frosting reduses pressure in the can, and your airbrush will just be spitting paint.
Just do yourself a favor, and invest in a compressor if you plan on doing much airbrushing.