Don't use chemicals on you grate. If there is lots of cooking residue you can use hot water and soap but I don't--water just adds to the problem.
Use your grill brush to remove the rust. Spray or mop with veg oil.
For my WSM I use my kettle to do a burn-off just before my cook. I'll light a full chimney of coals and save the top 10-20 for firing the WSM. The rest I dump into my kettle and cook both my WSM grates for a few minutes while I get the WSM ready to go. I then scrape the grates well with my brush and oil them before putting them in the WSM.
If I'm lighting a full ring on the WSM (i.e. not using the Minion) I do a burn-off on it.
Regardless, I always oil the grates before I cook and I try to after I cook (it's the times I can't that I resort to the above), after they've cooled a bit. We have lots of humidity and rain here--particularly this hurricane season. Heavy wind and rain turned my last 12-hour cook into an 18-hour. Didn't feel like doing much after the meat came off hence rusted grates the next day. But I scraped, dried, and oiled and they're looking good now.