Don't leave the old finish on. It is a laquer and has long out lived it's life.
You can use Air craft stripper or another method is the mechanical, I use a 36 grit sanding disc, not a grinding, 7", with an air grinder. Keeping the rpms down will help keep the heat down and help to keep the pad from loading up. After this I go over it with 80 grit on the da in the spin mode then on to 180 in oscillating.
Use a self etching primer on the bare metal then a urethane primer to do your sanding with.