The grit-blasting was completed today and revealed that the edge of the steel is rotted through in places.  This has not compromised the strength of the body but will do if it continues.  A possible solution is to paint it to protect it then plate over it with new Zintec to create a double thickness at this point, thoroughly sealed to prevent more corrosion although this does seem likely to have been caused by the unprotected aluminium that was used to clad the edge.  This cladding extended inwards by about 3-4in and was beaded with 1″ flat feathered bar so adding further steel may mean having to make a new cladding piece.