The thinner is your trick. Water will sorta work so will alcohol. I have had the best luck using Auto Air 4000 series "Airbrush Cleaner" .I always used Tamiya X20 a thinners for all acrylics, now that its hard to find I found this to be equal...Createx also makes this, as Createx is the mother company for Auto Air Technique is the same..higher pressure does help...unless thinned and not reduced...if you use reducer you will need a higher pressure
Here is a link for help AUTO AIR PDF info sheets...
http://www.autoaircolors.com/products/product_frames.html