There is a fiberglass filter that fits on the bottom. It's still available new.
I think the factory used some type of glue on the bottom cover. I was afraid of breaking the plastic on mine. I've heard of people breaking out the bottom and refilling them with charcole. Then you have to figure out a way to hold it in there without being visable from the outside. I just put a new fiberglass filter on mine and called it good. If I start smelling any fuel, I might look into replacing the charcole.
As far as refinishing them... The top of mine still had the original date code on it. I sanded on the sides with wet/dry paper to get rid of the scratches and rust stains. I masked off the top(to retain the date code) and sprayed the sides with SEM Trim Black. If I had to do it over again, I'd spray it with something that has slightly more gloss to it.
Do you still have your original hoses that go to the carb? One of the hoses should have either a red or yellow stripe on it. I reproduced the stripe with a paint pen.
