That may be residue from the plastic protective film that is usually on pickups when they are shipped from the factory.
Naptha is the solvent of choice for all guitar stuff. Effective and evaporates fast enough not to soften some delicate finishes.Would denatured alcohol take the finish off the pickup? Because it would certainly remove the residue...
You are the man Kev. As much as I used that stuff when I was building houses, I don't know that I would have stopped myself from using it here. I'd have probably paid for it too...Naptha is the solvent of choice for all guitar stuff. Effective and evaporates fast enough not to soften some delicate finishes.