For the cooking grates, i give them a good brushing with a wire brush, rub them down with vinegar a time or two, then coat them up with vegetable oil and start a nice fire to "season" them a little oil on the grates each time I cook keeps them well seasoned.
I had a few inside surface rust spots on my WSM when I rescued it. I just cleaned it up with degreaser (like simple green) rinsed it out well and coated it with vegetable oil. No recurring rust in two + years.
18.5 WSM, Green Weber Genesis320, Cast iron firepit(w/grill), and a Copper Weber 22" OTS