Protecting surface that has small surface rust already?

hm... depending on how bad the rust is (how deep, did it make pits or is it just like a smooth rusty finish). if its smooth-ish, id use like, 220, it wont scratch the metal quite as much and will make it easier to cover the scratches up. you use ETCHING primer (zombie) to cover it. first coat, kinda thin but wet, wait for it to flash (dry out most of the way), second coat, a little thicker, still wet, flash again, then you put the third coat on nice and wet. several thin coats will get inside the pits//scratches and wont bridge them and shrink exposing them when it really dries. really bad scratches, block sand them, does wonders, but if your e going to leave it at this stage of finishing, put some paint or something over it so the primer doesnt soak up water and let the rust come back.

the ket to primer application is, dont dust it on dry, that will cover so badly and look very crummy and not leave a smooth finish to start from when you sand it. you have to apply it like paint, wet and in coats to get the thickness to fill the scratches and such.