turtle71 there was a lot of prep work, my buddy that helped me with this is the owner of a very well respected body shop and he showed me a truck he was working on that had to have a new roof panel, the only thing that attached it the the rest of the body was this same glue. As far as the prep work goes, I took a small sand blaster and just keep going over the rust until it was all gone. I then primed the bare metal a can of self etching primer and painted it with a rattle can. Where the 3M adhesive was applied we used a grinder with a sanding disc on it to get the area rough. We then used self tapping screws to hold the patch panel down until it was cured, (24 hours) I also did the same thing on the bottom side so that there is no way for any water to get under the panel. It was not that hard to do, it just took some time.
The top panel is 16 gauge sheet metal. On the bottom I used lighter gauge because there was a lot more bending and shaping to get a good fit.
After 24 hours I removed all the screws and filled the holes with with the same 3M glue.