If you have access to a hand sander of some sort that is your best bet. Rough it up with 80 grit real good, then follow with 180 to knock out some of the deeper scratches. 2 coats of etch primer, 2 coats of a good catalyzed high build primer, sand smooth with 320, 400 (maybe 600 depending on color) then you'll be ready to paint. I've been doing grilles and other chrome stuff like this for a while now with no problem. Just like any other painting project, skimping on prep work will always turn up and bite you in the end.