How do you charge this felt pad with pumice? The videos show lathe wheel use, where you put the pumice-water mix on the glass, but I’m not sure that’s how to do it on a lap grinder. Thanks! Your site has so much good info!

It's very similar in that you'll mix your pumice with water to create a mud consistency and apply to the center of the felt pad with only a slow drip of water in the center of the pad. 

You'll want the grinder to move slower than you would for diamond grinding so you don't lose your pumice to centrifugal force. Work the piece in a random pattern across the pad and every so often pick up the piece and check for pumice. Re-apply to the disk as necessary.