I found one way that works (copied what someone somewhere wrote years ago) and stuck with it. If I deviate too much, it doesn't work, so I didn't bother to do too much other experimentation.
The shaking technique seems to be the whole secret for the poly powder I use. Things that have to be "right" are bullet to bb ratio, and powder ratio. Also, the residual powder and lead dust needs to be cleaned out periodically for best results, about every 15-20 batches for me. I use a sandwich container with snap flaps and silicone seal, and about three BBs deep. BB weight has made virtually no difference for me. No more than 80 .22 bullets and no more than 25 45/70-500 bullets at a time. "Toss" the mix so the contents slaps the lid and sides of the container mildly for about a full minute, then about half a dozen very hard, straight up/down shakes to really slam everything around. End on a down stop and let everything sit where it fell. Open lid and inspect. If there are lots of dings/pits showing lead, repeat the toss a few times and end end with the hard slams again. Eventually an even coat appears with very, very few defects. After picking the good ones, shake again a few times and pick more. Usually twice gets it, but near the end with few bullets the coating can get very fuzzy and thick when re-shaking, so be careful "overshaking" them. Pick with small needlenose pliers and tap every single one a couple of times hard on the lip of the container (hit with the hinge area of the pliers) to shake excess as a QC measure. Set on bases on non-stick foil stretched over a thin piece of sheet metal and bake.
The way I do it as described above, if you take a finished bullet out and wipe off the powder, the bullet surface will appear the color of freshly-broken cast iron and will have a surface texture similar to bead-blasted aluminum. I believe the abrasion of the oxide layer and the surface texture created by a brisk and extended shaking make for superior adhesion and depth of coating, as if the paint is actually embedded into the surface of the bullet.