Kevin, a mate of mine restores Corvettes & therefore does a lot of grafting new panel sections to old ones. To butt-join panels, his method is use bolts & nuts with large diameter washers to hold the panels together - just a half-hole in each panel for the bolt. Lots of fine tuning the alignment, then tighten up the nuts.
Next step is to grind back the edges of each panel in between the bolts - a chamfer of about 1" back from the edges. The V section formed by the 2 chamfers is then half filled with increasingly wider strips of f/glass mat/resin & consolidated with a roller. (Masking tape is often used under the joint to stop resin running thru).
Once these sections have hardened, he takes out the bolts & repeats the grinding/glassing process where the bolts were.
Then the rest of the V section is filled with continuous strips of glass/resin to about 1/8" above the panel surfaces. After plenty of time for curing, the excess is sanded off & you end up with a perfectly stable & very strong joint.
The choice of resin is an interesting one - so many theories ! My mate has found that ordinary polyester resin is ok on the early polyester-based Vettes if the panels are in good condition - if they are not so good, he finds that vinyl-ester resin gives a better bond. On his top-quality jobs, he now only uses the vinyl-ester. I am told that it is ok to use epoxy resin to bond polyester panels, but never to use polyester resin to bond expoxy panels.
Hope this helps.
Kind Regards,
Peter D.