If I was stripping it down, I'd use the earlier mentioned red rubber grease or silicone grease.
If I was just doing it in situ, I'd use brake fluid as it will easier get all around the piston etc.
Hope pistons suffer with crap build up rather than corrosion most of the time so a re-build is a bit of an over-kill every time it sticks
I tend to push them out [as far as I dare] one at a time, holding the other(s)back & applying brake fluid with a cotton bud or similar, push it back in & repeat a couple of times
do the same with the other piston(s)
Give it all a good clean with IPA or meths & stick the pads back in
Quicker to do than type up