I would start out with a coarser grit, then get a finer grit to go over it before painting
When I did my tailgate bezel I think i used 1000 grit to knock most of the texture down then used 2000 to get it smoother, you could used something coarser than 1000 though I just already had it, just sand the shit out of it till you get it smooth then make sure to clean it really good before painting
I recommend using wax and grease remover to clean the parts, just put some on a rag and wipe them down real good