[Will]

You really don't want to glass bead plastic. Try walnut shells or plastic bead if they really need to be blasted.

I have successfully repainted grilles in the past by doing nothing more than thoroughly cleaning/degreasing them then shooting paint. Of course, the grilles were in good condition and the factory paint was still adhering very well. If the paint that's on them is flaking/peeling or badly chipped then obviously it should be stripped to look best.