I use ONLY the blade, sod using sandpaper that falls apart in use and puts rough sand down your bores or oil ways etc.

Im not aware of any reliable way of cleaning down the side of a piston while its in the bore.

If you want it PROPERLY clean and flat, then take it out and get the block skimmed, anything else is not going to get as good results.

If you arent goign to take it out and strip it then I reckon stick to the blade cause if anything goes down your oil ways or bores etc, you dont have a way of getting it out again without removing the block and stripping it, and if you are going to do that then you may as well skim it!