The little bit of crap/grit that tries to pass by the piston, on it's way to the exit hole marked in red, will also dig into the aluminum and score it a bit.
Cleaning and buffing off/crocus cloth the piston is good... but unless you also remove the aluminum scratches you will never restore a slick surface and it never works as well as it should. It is a bit of an inspection and maintenance item if you want it to work reliably.
Part 36 circlip on the rear shocks diagram; for the Mark 3 rear wheel change the brake caliper bracket needs to be loosened for removal by loosening the lower right shock bolt nut. The circlip would help to limit backing off the nut.
Removing and re-installing the Mark 3 rear wheel is almost a 2-man job.