[Preacher]

Bob, I think the biggest thing to overcome is matching the right topcoat over what you have existing so you don't end up with one finish attacking the other.
The problem with a test is you need to match Taylor finish exactly, and then see if the new coat will not attach the sharpie or the existing finish. And any kind of finish that has Acetone or Methylketone or something like that will mar the Sharpie.

One thing I would maybe recommend is using a clear static stick film over the signatures. They have clear pieces that you can static stick to windows and such and that should protect the sigs and be replaceable as well. Just cut to fit and apply...