The problem with these grille surround is there are so many road nicks that go pretty deep.
You can spot the deap one on the back side of the piece buy looking for outward bumps. You've have to clean/steel wool the inside to spot them. They are easier for me to see before you remove the anodizing. I pop the deep ones out with a rounded off punch. Most of the time with a $3 Harbor Freight Center Punch that I have grinded and polished the point to a bull nose. I pop them out. Then sand or file flat.
My other favorite tool is a Radiator Hose Pick to get in hard to reach areas. I don't use the point. I just use the side of the point. You don't hit it hard, it's just aluminum. Just multiple soft blows against a block of wood:
I like to file the straight pieces. The file becomes a "long board" and takes the waves from sanding and straighening out. The dull spot are the low spots.
The sands with a hard foam block. Or stick of wood on flatter surfaces.