Let a blister/callous form. Problem solved.
that is the only way it happened for me. I stopped learning throwing backhand and just used FH exclusively, which helped me learn FH quick but I am just now learning to throw backhand decently. Anyway, I had a nasty blister that formed and when it sunk down a bit I just covered it in duct tape. Athletic tape or any other tape I had would slide off. Duct tape stinks though, so if you find something better I'd suggest that.