Reuben wrote:

I have a 1983 VW Passat GL5 which is in reasonable condition barring
some serious rust on the training edges of the doors and the tailgate.
Professional repairers seem reluctant to take on repairs like this -
they want to replace the doors (possible) and the tailgate (not
possible to buy new tailgate now).
I'm considering doing the repairs myself and I'd like to hear from
anyone who has experience of this kind of job in relation to materials
and methods.
I'm realistic about what I can expect to achieve - I just want a
passable job to keep the car legal. If it turns out really well,
that's a bonus.

As others have said, by far the best result will be obtained by
replacing the doors and tailgate. Phone around your local scrappies.
Someone will have a suitable donor with reasonable panels.

--
Grunff