A couple of years ago I looked into a clam hinge for my 2011 Elise. The BOE solution doesn't work because the new-style clam drops lower in the rear and would collide with the exhaust and other components. I developed my own hinge and mounted it to the rear of the clam trunk floor using a part made from a fiberglass and carbon lay up. The layers went like this:
1. carbon (90)
2 - 4. glass (45, 90, 45)
5. carbon (45)
6 - 7. glass (90, 90)
8. carbon (45)
9 - 11. glass (45, 90, 45)
12. carbon (90)
The weave direction was rotated to either 45° or 90° to provide greater stiffness and resist warping. Here is a picture of one of two molds I made to match the trunk floor at the hinge location:
I used three layers of fiberglass to test the mold before making the actual part. Here is a picture of the completed parts:
Next is a pic of a test fit of the hinge mount on the trunk floor and a pic of the clam open on the hinges:
So what's this all got to do with body repair? Glad you asked . . .
When I cut into the trunk floor to create an opening for the hinge assembly, I got to examine the structure of the fiberglass. The material has a very hard and thin skin on both sides (interior and exterior) that is resin-rich. The core is much different though. It is composed of chopped fibers that are weakly held together with a very small amount of resin. If I had laid up this part, I would have called the core resin-starved. The fibers were easily pulled apart with an X-acto blade or tweezers. The material was weak enough that I decided my hinge mount needed to be not only bonded to the trunk floor but mechanically fastened through both the interior and exterior skins of the material. This would help distribute the loads imposed by the hinge throughout the material rather than concentrating them on one hard skin that could be peeled off the core.
I think the structure of the fiberglass I examined helps explain why body damage on an Elise or Exige is often so extensive. Once the hard skin of the laminate is damaged, the part loses most of its strength and is easily collapsed by point loads. One of the challenges in repairing a part - rather than replacing it - is to create a repair that does not introduce new problems. For example, if a repair is more rigid than the original part, failure can occur on the edge of the repair when the whole part moves. This can lead to cracks in paint and resin. One way around this is to feather the thickness of repairs out over a large enough distance so that the stiffness of the repair drops back down to that of the original part. This is easily done with patches of increasingly larger and thinner layers of woven cloth. It can be done with mats of varying weights, but it is - in my experience - harder to do because mat has a lot more variance in thickness.
Most of my fiberglass experience is with model aircraft up to quarter-scale size. An original layup for a part might consist of several layers of 6 ounce (per square yard) cloth, followed by one or two layers of 2 oz. cloth. A finishing layer of 3/4 oz. cloth - with the weight and weave of a fine silk - further reduces stress risers and helps create a very smooth base for the final finishing techniques. For a patch on body work, the thick cloth would go on the back and as the weight of cloth was reduced, the patch would get larger to spread the load out and reduce stress risers. Depending on the contours and stress on the area of concern, something less than 2 ounce cloth and maybe even just 3/4 ounce cloth on the exterior surface could be used to rebuild the exterior shell of the soft-core composite originally provided by Lotus.