I had a similar problem with my 84 190E, I slowed it down but could never stop it. Every year I would remove rust and recoat the area until the day I traded the car in.
I'd suggest looking at
www.eastwoodco.com for restoration products. They offer products designed to stop rust and restore the original finish. The other option would be to find a resto shop or body shop that know's how to deal with this.
Good luck and make sure you check the area on a regular basis to insure it doesn't reappear.