What’s a good black paint to use over the hummer h2 grill guard since mines starting to rust

 

These are powdercoated, not painted. So you could re-powder, but it is pricey.

If you need to paint, and you want to keep a smooth finish, satin (matte) or semi-gloss tremclad black rust paint. Only stuff for outdoor exposure on metal.

if you want it rough, you could always use bedliner, like Rhino or similar. But I would not do that, it looks terrible. The Jeep guys like that stuff.