Use oven cleaner to remove grease and Wd40 for the rust.
We have a big plastic bin-the kind that comes with a lid--and we place the grates in the bin, fill with warm water and Dawn dish soap. We let this sit for a few hours and then use a wire grill brush to clean them off. The wire brush should take off the rust as well.
I'd soak them in ammonia. Put them in a garbage bag, pour in ammonia to cover the grates, then leave for a couple days. You may have to go over them lightly with a Brillo type pad after, but that should do it.
Spray with Jasco premium paint and epoxy remover, soak 3 hrs, wipe off.
Spray with Ospho and soak overnight for rust removal.(turns rust back to metal)
Remove residue with grill brush. Rinse and wipe off.
***If wife not available for above. Go to Walmart and get replacement non-rusting
Stainless steel grates for $15.00. Throw away old, place new one in.
wife like the wal mart subjection
Be careful if you use any of these toxic chemicals to remove the grease and rust, be sure that all of it is rinsed off completely. I soak the grates in Dawn dish detergent, then scrub with a Brillo pad or the scrub side of a sponge.
Eventually, I bought new grates, like Florida Beachpotato, and tried those copper grill mats--much easier cleanup.
I also now appreciate the copper grill mats. I did note the replacement grills have almost doubled in price in the last 2 yrs. Oh well. Look this site over too.
***Ellis is spot on about chemical residue, needs to be cleaned completely from the grating and rinsed well. Then do it again. Heating chemical residue is dangerous to breath and it will transfer to the food spoiling the taste and of course being toxic.