Funny you should mention 3M Jim, I contacted them the other day, just awaiting their reply.  Will post their recommendation when it arrives.  So far, from 2 other adhesive manufacturers, I've had similar advice, i.e. an industrial strength, high heat resistance contact adhesive.  However, one of them suggested using etch primer paint on the glued faces before gluing. Their idea was that this will prevent the ally corroding.  On the other hand, the first firm said that the glue would do that anyway. I'm tending towards thinking that probably water seeped in between the ally and the ply due to the less than thorough way in which it was applied.  More care over this stage should pay dividends later.