the full tech sheet on the 421-17. You probably have the general purpose 1.8mm tip that was pretty much strandard on the older gravity feed guns so that should be OK. Make sure your gun pressure (35-45 psig) is set AT THE GUN. I see the pot life is only an hour, similar to the PPG surfacer that I use. Don't let it sit around mixed before using. Spray it as soon as you mix it. As the pot life approaches and is exceeded, the stuff thickens up rather fast. That in itself could lead to spray problems with any gun.