The underside door trims of the Bmw E34 are a rust nest.
If you see some rust spots in this area, you should consider
changing all four trims (BMA
Remove the old trims out of the doors by popping them off
at one end.
Clean up the surface. If it is rusty, remove the rust with
sandpaper, apply some rustproof liquid and paint the area.
You can wax the area if you want, then put the new trims