Sanding is permitted at most do it yourself yards as long as you use a dust free sander and a HEPA vacuum.
Sanding depends on the existing bottom paint. On most ablative bottom paint, a through pressure wash is all that is necessary. The waterline area may have some oil or other contaminants preventing a good bond. I will wipe the waterline area with paint thinner compatible with the new paint to remove oily residue.
If the waterline area is in bad shape, I will sand that area only.
Right before applying new paint, I wipe the entire bottom with liquid sandpaper with a clean cloth, to clean and soften the old paint to promote better adhesion with the new paint.
I paint the prop, shaft, keel shoe and stern bearing housing with bottom paint with great success. I prime the bronze with Interlux Interprotect 200 epoxy barrier coat to prime and electrically isolate the copper in the bottom paint from the bronze hardware.
Without the epoxy barrier coat isolating the copper in the paint from the bronze, the copper in the bottom paint will wear away quickly.
I get about 4 years from painting the prop. My prop spins at about 600 to 700 RPM so the paint wears at a slower rate. I do apply 3 to 4 coats of bottom paint on the prop.
Interlux's Paint Guide and web site describes the painting of underwater metals using an epoxy barrier coat in great detail.