sand paper

Walmart or auto supply stores in the car body work dept.

HF also sells a package of 220, 500, 1000, and 1200.

I don't go below 320, but do use 320 a good bit.  For me it seems that 320 cuts better than 400, but cleans up just as well with 600 grip.  I finish up with 1200 and then 2000 both wet.  Ends up scratch free.

It's the number of the grit that will matter. I typically use 400, 600, 800, 1000 and higher grits. Any lower numbers like 150, 200, 320 will create scratches that are almost too deep to polish out. I get my sandpaper at Walmart's automotive department.