Sounds like you might have plugged the vent on the siphon feed thus "pressurizing" the can. I'll second the gravity feed recommendation, especially on a metallic, don't have any experience with the HF gun though.

If you are using a metallic (single stage, not bc/cc?) you won't be able to just sand off the run and polish it and have it look "right". When you sand a metallic you not only sand the paint film you also sand the suspended flake, changing it's shape. You'll end up with, for want of a better word, blotches. The other comment would be about doing a metallic, especially if it's truly a candy (although that word is also used to decribe a brilliance rather than the traditional transparent which is more what candy means to us old timers)one panel at a time will likely result in a very uneven appearance. The key to getting an even looking finish is consistent coverage over an entire "face", say the entire side, or the top surface. You get a little leway say, hood to roof to decklid because they're separated by glass, but fender, to door, to quarter panel, is far less forgiving.