have an audi a3 3dr but she has some rust on the bottom of each door only approx half an inch in height just above the black plastic runner - is this treatable at home by self what would I need to do ? any serious suggestions welcome

 

Pretty common problem on the A3's. You'll probasbly find under the plastic runner will be rotten.

May be worth going to the scrappers and trying to find a new door.