Fingerprints don't matter much. Shake the geezus out of them and the powder will stick. About one full minute of firm shaking like a percussion shaker, then hard as you can up and down about ten times, ending on a firm downstroke. Pull the lid and look to see how many bare "polka dots" the bullets have. If you see more than one or two on each bullet, repeat. If you start getting warts of clumped powder on the bullets, you used too much powder in the mix but a light shaking will knock most of them off. Humidity is never a problem for me and my shop gets nearly 100% like a sauna sometimes.
The way I do it, static electricity is less of the mechanism and embedding the powder into the bullet's surface is a lot more of the mechanism that gets it to stick. If I take a coated bullet and wipe off all the powder, the bullet's surface has a very fine bead-blasted look to it, like matte aluminum. The powder is abrasive and the hard shaking blasts off the surface oxide layer from the bullet and really makes a nice, textured surface for the powder to adhere.