Can anyone tell me how I would go about prepping this plastic fork guard from my motorcycle without the polyester tooling gel coat ruining the piece?  Painting it is not an option.

To be honest I actually think waxing increases the risk of damaging the part. Wax works well on compatible surfaces like 2k paints where all you are trying to do is stop the resin from sticking. But on incompatible parts, wax isn't going to do anything to prevent the resin from attacking the surface. One PVA will do that, and the wax might cause the PVA to fisheye leaving the surface exposed for the wax resin to attack. Food for thought anyway.