My personal opinion with barbecues, although I doubt everyone shares this opinion, is that it doesn't matter if they are a bit rusty or dirty.
If there is so much rust that you are worried about the structural integrity of it then I don't think rust converter/hammerite or anything will do any good. Removing the rust will leave you with a thinner bit of metal than you started with. Thin metal + heat is not good when the metal is all that is preventing your coals landing onto your sandled feet.
If it is just for aesthetics, then be a real man and show your barbecue is a battle-proven old wardog. People will respect you for it. I nearly lost all respect for a friend of mine when 20 minutes after we finished eating him and his fiancee removed parts of the barbecue AND WASHED THEM!!!
And instead of sitting around the barbecue to keep us warm in the garden until late into the evening, drinking, talking and burning stuff, they put the hose on it (which will probably lead to premature degredation of the barbecue, I might add!)
Even the vegetarian I had with me accepted that washing a barbecue grill is a heinous crime and should be cleansed only through the fire that will soon die down to cook the food, and the wire brush that removes all bits of crud that show themselves as too weak to remain as part of the barbecue.
It's not a way to cook food, it is a continuation of pagan rites that have existed since the dawn of time.