I always have believed in the saying "when in doubt, do nothing".
You didn't make the marks, so you really don't want to own them.
There have been occasions in some situations over the years where there was a problem created by someone else and I was given written permission by the client to "experiment".
I have had many successes while doing this and I have often been able to pull a rabbit out of my hat.
It has always been in a situation where I was told by the client "we have nothing to lose by trying".
I once got a group of limited edition sepia prints of Native Americans from early Arizona history.
They were unframed and had gotten damp and had large areas of black mold on them.
They were not irreplaceable and the client didn't want to send them to a conservator.
I was given permission by the client to try to get rid of the mold and restore them and frame them.
I was able to restore them to pristine original condition and mat and frame them.
Many of these techniques have been put in my brain file to be possibly used again.
I would not suggest to someone who is not experienced to "experiment".
I have been framing for 44 years and again I stress, "when in doubt, do nothing".
You won't go wrong with that attitude.

Good luck with your project.
