Hey there Thomas, My airbrush broke last night and think its terminal, had it apart loads of times and think its finally given up the ghost this time!? Can't afford another so don't know what I'm gonna do? Its one of the very first ones they did and I don't think its as robust as the newer ones.
As for the windex, I'm not 100 percent shure I'm just going off what other peaple have said. Though the emulsion paint is very very thick and needs thining quite a lot!! if you did this with just tap water it would just bead up on the body and be a real mess. The windex acts as a flow improver and as a surfactant stopping the thined paint from beading on the body. Hope this helps.