Hey catso,
Well after having one ex army 90 defender one 1959 series 2 and a series 3 and restoring them back to original , i had a similar problem. Due to landrovers having aluminium doors etc they always seem to corrode in the most impossible places, the only way ive found is to take the paint right off around the area and go back to bare alloy and use acid etch primer to seal the blemish.
I think its where they sometimes dont have a good covering of primer before painting and maybe the paint is too thin for british types of weather.
But unfortunatly there isnt much you can do once its starts , only maybe to prevent it happening again for a while.
There used to be aplace i went to in bedfordshire called universal salvage and the amount of new doors/shells and chassis what they had stock piled up due to similar problem before they started.
My army 90 had 17 coats of paint on it and it was a pig to take off, but that didnt have alot of corroding on it , and i was surprised.
New door skins for your disco would be very exspensive , but if you dont mind paying then ask a decent bodyshop or coachworks.
I know the people here very well.....
http://www.rogersofbedford.co.uk/defaultnew.ht ml
Sorry i cant help more :)