Good evening.
Another thing to do while you're there is to make sure the calipers are sliding properly on their pins. Use Rubber lube...Where would we be without it!
For cleanig pistons in situ, I have found nothing better that parcel banding tape. That's the fibrous white stuff you always cut your finger on if you lift a heavy parcel by it
It can be torn into very thin strips, & with a good silicone lubricant does an excellent job of cleaning the pistons, & being thin can get right round caliper pistons that have little tolerance between them & the caliper body.
Second favourite for this task, is a cable tie.
Cheers.