I have sanded out scratches and minor pitting in stainless and aluminum to take them to near mirror shine. By minor, I mean they were no more than 0.02" deep and mostly 0.005" or less. I have started with sandpaper as coarse as 220 for the nastiest of the pits or scratches. Otherwise starting with 400 is a good place to start. I have never sanded out an aluminum radiator and that is up to you. Trim and such does not hold a hot pressurized fluid like a radiator. If your pits are close to 0.005" deep (about the thickness of a sheet of copy paper)...it's probably OK to sand them out. What you need to be very cautious sanding at the corners or breaks. Those areas will be thinner by nature and you don't want to make them even thinner. Just don't sand areas that don't need it.
Wet sand with 400 until the pit/scratch is no longer visible. By scuffing it up with the sand paper, it may disappear to the eye, but still be there. Then wet sand with 600. The flaw may reappear as the 600 brings back some of the h surface. If 600 does not remove the flaw within you available patience, step back to the 400 then back to the 600...then 800. 600 or 800 should give you a natural-like finish, dull, but rather smooth. If going for a reflective shine, move on to 1000, then 2000, then polish it with something like Mother's. Spot sanding out pits will leave low spots that will show back up once the reflective shine is brought back. If ripples or divots end up being too noticeable due to the shine, you can go back to to the 600 or 800 and dull it back down. Keep in mind the more you sand the thinner the aluminum gets.
If you take to sanding on it, just know you will be sanding and rubbing for longer than you think.