In article , "Reuben"
says...
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.

You will need:
angle grinder with a selection of disks, MIG welder
tin snips, hammer, pliers, workbench
sheet(s) of steel
rust treatment, filler, paint, sanding block and abrasive sheets

First run a coarse linishing disk over the crusty areas to remove all
flaking paint and rust until you reach something vaguely resembling
steel. If the edges of the frame are still mostly there you can just
treat them with Kurust, if not you will need to weld in patches to
restore the shape of the edge. Tidy up the hole in the skin with snips,
cutting back until you reach something that looks like it will weld.
Make up a patch using similar weight steel (something like 26SWG?) and
fold the edges that sit on the frame to about 90 degrees so it sits
snugly. Tuck under the cut-away skin and tack-weld securely in place -
don't try to run a seam weld as you'll probably end up with lots of
distortion and burning. Linish any blobs of weld so you can hide them
under a thin skim of filler, but be careful not to go through the skin.
Fold the edges tightly around the frame (no need to weld). Treat all
the exposed edges with Kurust. Wipe a thin layer of polyester filler
over the weld and sand it smooth. Paint.