I agree, the old siphon gun is the best way to do it and use a large nozzle, but I usually spray color flake over a similar base color as it will give you a more uniform finish. I usually spray large flake, but I also add smaller flake of the same color to help fill. I have sprayed color flake over a black base with a 50% coverage which gives a different look to the finish. Gravity guns usually plug when spraying flake.