Unless you just want to experiment, I see absolutely no reason to do anything to 845 except add a second coat. For the last few years I've done my winter preps in November with 2 apps of 845. I still had beading in March after a PA winter on a non-garaged DD. This year I did 2 coats of 476 on the same 2 cars and I expect I'd have to chisel the stuff off if I needed to remove it.

Bill