Once upon a time there was a kit car, I forget who, that used some panels glued to the car door to subtly change the body lines and blend the new nose with the new tail section. The tail section was also glued to the unibody.
OK, I understand the making of the new panel and the gluing, but what about the edges? How do you finish the edges so it looks good and won't crack or separate between the glass and the metal?
The same thing would go for the trunk lid.
I would like to assume sand the edges down to a radius of your liking and apply a skim coat of plastic filler and re-sand. Possibly mixing up a small batch of fiber glass resin to take care of any frayed, exposed fibers after the initial sanding, and before the Bondo.
I'm not sure that would take care of an future separation between the metal and glass.
I'd bond the parts w/ a structural adhesive to metal that's been gone over w/ a 40 grit disc, add countersunk-head pop rivets, feather the edge w/ an 80 grit disc then skim the joint with "gorilla hair" which is body filler w/ fiberglass strands. Grind that w/ 80 grit then skim w/ regular filler, shape, glazing putty, block, high build primer, block, primer, guide coat, block, sealer, paint.
I purchased a "kit car," pre-finished.
It was a C-4 corvette with a "body kit" to make it look like a C-5. Other than the car already being fiberglass, sounds just like what you are describing.
There was a new nose/tail, and door skins.
The door skins were bonded(glued) to the existing doors and the edges were feathered with bondo, then painted.
With all the abuse I put the car through, the bondo started to lift, and the paint above cracked.
I did not assemble the kit, it may have been done correctly, however with the abuse I put the car through, any repeated flex of the inner (original panel) that did not match the new skin would cause problems over time no matter how properly done. Including flex from temperature variations, slamming doors, and shrinkage of the glass drying out over time.
I don't have a great picture, but you can almost see the split, if you squint and your imagination is strong.
Bottom edge of door, "vertical" part, you can see a white line, this was the split bondo covered with paint. The horizontal line is just dirty, the glass door skin overlapped.
IMO, I feel this is the problem with kits, they are designed to look cool, without any real thought to long term durability and quality. I can't think of any way to keep glass bonded to metal with a feathered edge and no future separation/splitting/lifting etc. But then again, I never went to auto-body school.
Bonding, either with or without feathering, seems to be way easier than cutting the edge of the metal back and then using a glass piece with a prefinished edge, but that way would seem to give a better look.
Of course on things like doors you'd have the back edge to be filled and finished, but at least any cracks would be out of sight until the door was opened.
For what I want to do if I can't find a way to make it look good AND be durable then it's just not worth doing.
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