We are having a room with high walls repainted in a deep color. The
same wall has a typical return-air grille (24 x 24") with adjustable
vanes that has the usual factory-baked enamel, off-white color. We
wish to repaint the grille, so that it blends into the wall color,
rather than standing out in stark contrast.

It would seem that using an enamel or gloss paint is necessary, so the
grille may be cleaned periodically. Would it be best to use a primer?
Should the grille be sanded first? To achieve a smooth finish, should
the grille be sprayed?

What is the best way to do this job?

Any advice appreciated.

Thanks.