You need the water trap if you are painting for a long time , like 2 hours or more. If water gets in your line from the compressor , it's because your comp is working hard and creating hot air then it cools in your line which makes condensation. It will ruin a painting. If it an old compressor that has built up rust in the tank or pipe fittings the little pieces or rust break off and will reak havoc on your brush causing spitting of air and you may never get it out of your brush!! I highly recommend at the very least a pistol grip filter no matter what!! It is the little filter that goes directly on the brush between it and the hose. They are worth every penny. Hope this helps.
I would want some more protection if I were you. I would at the very least till you get a trap cut a 2 inch square piece of some panty hose, or ladies nylon and place it over the bottom of your airbrush and thread the hose over it, this will act as a crappy filter, then trim off the excess panty hose. This will at least stop any debris from getting up in your airbrush TEMPORARILY!! I would check it after you are done painting and see if anything gets caught in it.