I am thinking about doing some customization of the rear clip (around the tire/gas cap area). The material on that piece of body work seems much more rigid than the front/rear bumper covers.

What is the generally accepted material to use if I were fastening and/or creating some fiberglass pieces. I have seen pictures of people installing widebody kits on the rear shoulders with what appears be standard fiberglass matt and resin.

The "rear clip" as it's commonly referred to - notchback or fastback - is made out of SMC. The hood, headlight doors, decklid, and roof panel are also made out of SMC. It is very similar to fiberglass, but unlike the RIM panels designed to flex under loads and resist dents - the fascias, doors, and fenders - SMC is not designed to flex and can chip and break like standard fiberglass.

You will have to use a proper SMC bonding agent to properly perform the type of modifications you want to perform. Standard fiberglass resins and mat can be used under the areas not to be seen to assist with structural integrity, but whatever ultimately lies on "top" that is seen should be an SMC-style agent.

SMC (sheet molded composite) has a mold release agent built into the material to help parts be removed from the mold easily under mass production conditions. Normal fiberglass resin won't adhere to SMC and will eventually fail because of this release agent. Proper SMC resin must be used if the work you are planning to do is going to last.
Or at least that is what I remember reading some time ago.

Other products that may be of help;

http://www.pliogripbyvalvol...SMC_Panel_Repair.pdf

http://3mcollision.com/prod...-adhesive-08219.html

Thanks guys. I have a can of SMC filler/adhesive so I am good there. I am free though to build my modification in fiberglass and use the SMC for final attachment, filling, sanding though correct?