I have never cleaned the exterior of my WSMs... those are marks of pride! I have only been using my new 22 lately because the 18 is so far ahead.
If the grates are rusted, you just have to srcrub them down with a wire brush and oil them to prevent the rust from getting out of control. I would replace badly rusted grates.
Best way to clean the grates is to cook off the residues immediately when you are done cooking. If I know I am going to have the performer fired up anytime soon, I throw the grates on there for cleaning later. Otherwise, I take the top & middle sections off the WSM, put the grates on top of the charcoal ring, and put either the lid to the WSM, or now on my 22 WSM the lid from a performer and let the stuff burn off. I like the performer lid because I can shut it afterwards and have a great seal all the way around to shut down the coals.
Then just hit them with a wire brush to get rid of the charred residue. Wipe down with a cloth and they will be shiny.