Are you drying the filters before putting them in tin cups? Initially I dried GF/D
filters in the tin cups before scruching them closed but had a lot of problems
with a white depost which I assume was the acid reacting with the tin.
Instead I dry the filters pinned onto polystyrene board at 40C for a couple of
hours, then seal up in tin cups and have had no problems with corrosion. I
also switched to use Whatmans 41 as this causes fewer problems for our mass
Hope this helps