From the pictures on page one there was no way of telling what was hiding behind the screen. However once the screen was removed the damage could be seen! but the first thing to do is remove the screen and because the rubber degrades and causes the damage in the first place the easiest way to get the screen out is to cut thought the rubber and fit a new one when complete. The window screen is toughened glass so can take quite a lot of pressure but even so be careful and get help (if it breaks it will literally explode).
Also to properly carry out this repair the dashboard has to be removed, this is held in place at the A- posts and along the back of the dash on pieces of angled plate as seen in the picture, if the area is very badly rusted it is possible for the dash mounting angle to be lose from the front panel in which case I suggest drinking heavily.
After removing the screen (and placing it in a nice save place!) and removing the dash (check your Haynes manual for all the fixings) I have then cleaned back the paint work to reveal what damage is present, and to be honest it isn't that bad! the key point here is to take your time and try not to remove any of the good metal, I find a "Scotch 3M" cleaning pad in an electric drill excellent for this.
To repair the damage I decided to firstly repair the outer panel, then repair the inside panel, as I always like to try and keep as much of the original metal in place so as to keep the shape correct (you can repair the front panel and make sure it fits to what is remaining of the rear panel). The repair panels came from Classic Volkswagen Restoration (or Alan Schofield as the man behind the establishment!). The repair panels (3 in total) are enough to do all the bottom of the screen on the outside and then other panels are available to repair the inside.
