if you can leave them coupled to the bike and pump them out ( you may need to top up your resevoir )
A trick here to get stubbon pistons moving is to leave the caliper in position but take out the pads on the half of the caliper that is stuck then the free side will not pump out first
once out you could use a rotary wire brush in your drill i use a zip wheel in a grinder if there terrible though with big pits you need new pistons but hey its worth a try
either that or 600 wet and dry and clean them up once clean grease well with rubber grease
( this type of grease doesn`t degrade the seals any
)
the first method is more a last resort really but id sooner take a caliper to bits and repair it for an hour than replace it for myself
for a customer id replace it that way down the road any problems and you dont get called if your so called repair suddenly goes wrong its down to the new part and not you
if they are silly money like ive done in the past i will overhaul them though