This is Michael Knight at CASTALDO.
VLT should work very well.
Two small cautions, however. It’s always a good ideas when tackling
anything new or out of the ballpark to try it out on a small scrap
or waste piece. You never know. If you have something similar or a
crumbled piece of the fossil ed bone, that would be great.
The other concern is if the fossil is porous. The rubber might flow
into the pros and make it hard to remove. Or get stuck there
The so first check it out with a strong magnifying glass. If you see
pores or have any doubts, you can coat the fossil with some sort of
barrier. Spray polyurethane from the hardware store works. Maybe you
can find something else that is compatible with fossilized mastodon