Welchen Schleifer soll ich für diese Wände verwenden?

I skipped sanding when I did my garage. I applied a coat of primer and painted over once dried. Two years later it looks brand new.

I’m not sure what you are going to do by sanding that, at least it will not be less bumpy.

Also, the paint will just pull off after a while because of moisture. I would leave it as is.