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Evercoat has a nice line of SMC resins (for use with standard fiberglass mat, cloth, etc), adhesives and even a nice fiber-reinforced filler. I'd take a look at them, I just used their products to repair similar damage to my front wheelhouse.
The rear tub is made of S.M.C. it is cracked and splintered, just asking for the best product to make repair.
I was just wondering if you were experiencing cracking, splintering, or seperation from the bonding seams.
As Dope mentioned, Evercoat has a great line of products available.
I also highly recommend Valvoline's Pliogrip and 3M's Duramix line as well.