I can't help you with the build but for the rust, use barkeeper's friend and some elbow grease. I can't tell how rough that surface is due to rust and welding but you might can just use a scrub pad to get that off of there(and barkeeper's friend, of course). Depending on how bad it is, you might want to be careful putting weight on it.
Also, I wouldn't paint over that before you sand it or something. With that surface chipping and coming off like it is, the paint probably won't hold.
Best of luck to you