Hey AC,
While I believe in voltage control through use of either the box or Cannon's built in Positive Ion Control, coating your downrigger weights is neither necessary nor effective.
My own experience indicates that the coating invariably cracks, allowing water in and therefore allowing electrolysis to take place as before.
A more effective step is to use Cannon's Terminator or other non-metallic snaps to insulate the ball from the wire, breaking the circuit and taking the ball out if the voltage control equation.
Further, I'm going to play around with the zero-voltage low drag braids this summer, fishing it side-by-side with Positive Ion Controlled wire and I hope to gain a better grasp of the "braid vs wire" debate.
Regarding braid, the only type worth considering is the low drag variety that has a smaller diameter than wire. When Spectra downrigger line first arrived on the market, it was a larger diameter than standard 150 lb test wire and blew back farther rendering it less effective than the tried and true stainless.
Contrary to popular belief, braid can be used on Cannon downriggers and the quick-change spool feature on the new Cannons makes it easy to switch and run a test of your own!
Good luck!