On my 94 SL2, I used a rust converter liquid called Good Stuff. It is a
harsh chemical that will safely kill active rust and convert it to black
primer. You can find it many auto parts places. Great for killing and
preparing basic surface rust for repainting.

If you are referring to the bottom of the rear dooor, inside the door wheere
the step plate is, I had the same thing. Bad bad Saturn for never correcting
this on S-series models. Keep on top of it with Good stuff and touch-up
paint. Make sure to allow each application of Good stuff to dry a few hrs.

marx404