I would think that rough sandpaper would remove all the old paint and give you a good, rough surface to bond to.

Minwax High Performance Wood Filler is essentially the same as "Bondo" used for autos, but in a different color. Since it's used for dent repairs in autos, does that change your mind about it at all?

According to Miniwax website directions:

Area must be clean, dry and free from paint, varnish, dirt, and grease.

wire brush?

"Minwax High Performance Wood Filler is essentially the same as "Bondo" used for autos, but in a different color. Since it's used for dent repairs in autos, does that change your mind about it at all?"

Not exactly.
Bondo has a bunch of added 'proprietary resins' that make it set up hard and match its expansion characteristics to metal.
Wood barely moves with temperature, but the humidity changes the moisture content i the wood and makes it move hundreds of time more than metal.