cleaning grills

I know with a Drum it’s very little cleanup as the drippings end up on the coals. With a vertical offset smoker how often if at all do you clean the interior of the box?

Anyone have an easy method of removing rust from a grill grate and water pan. Tried a drill with wire metal brush attached and still cant get it all off. This is a home made smoker I aquired from a family friend so the size is not standard and cant find replacement grates.