You can sand it down, clean it up but it's really just a short term patch. To really fix the problem you need to separate the aluminum skin for the door repaint and seal the steel frame and re-install. Did it with the defender and actually hot dipped the frames which solved all the issue. It is really hard and time consuming to fix this so I just picked up a complete set of extra doors for the disco for $85.00 and will swap them out when needed. It can be done but it's hard and no fun.
Best of luck