Mike - I’m no expert in the painting realm, but it sounds like there is a contaminant present that isn’t being removed by sanding and degreaser/wax removers.
Could be something containing silicone was spilled on the fender, or maybe on cloths used in the cleaning process have made contact and left residue on the fender.
Not sure what might be used to decontaminate the fender, when stripping it down again, but reading other Bolters comments, it’s clear that it’s a very difficult process.
Others should be along to offer more insight.
