I hate to admit but I think the only fix will be to file finish (for a bloody long time) and 2-pak paint the colour of your choice.
Another alternative to consider for diecast grilles is spray on chrome. Not sure how well it would stand up to the usual assault from gravel and stones, etc, but given that electroplaters are avoiding diecast it may be worth inquiring about.
Here's a mob in SA who say:
ChromeTek
A few years ago we came across a new and interesting product in the United States. It is what can be best described as a spray-on-chrome. We have watched the development of this product in the United States and now that we have been satisfied with its resistance to the elements and its longevity, we are proud to be able to offer this service in Australia.GM Restorations
0427 098 216
grant@gmrestos.com.au
http://www.gmrestos.com.au/?ChromeTek