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