Honestly I would use trim black that comes in a spray can from SEM or the like.. Clean the grill with a dawn dish detergent and a red scotch brite pad until it does not bead an water anywhere..
Then just shoot the grill with the spray can.. couple coats will fill the scratches and do the job..
Ive went to the extreme on some of these applications.. What I mean by extreme.. Adpro.. epoxy.. single stage with flattener.. or an activated base or flatted clear..
For the grill.. Probably overkill for most applications.. Now for a high dollar show car.. I would go to the extreme that way years from now, the customer would not come back and say that something had faded or is not holding up etc..
Food for thought..
Sometimes, we really over think the small stuff because we get so used to the way we address everything else on these cars..
If you are looking for something that will last as the clear on the outside of the panels, then go the extreme route..
Maybe I should not call it extreme, it just seems extreme vs shaking a rattle can and spraying lol