Those are not sand dollars. They are called capiz

shells.

Thank you for the clarification about the type of shell. I already have a mask but I dont want to grind the shells, I'm trying to preserve the shells at all cost.

Hopefully someone will have a tried and true method to share .... I’m just kind of thinking out loud here — I wonder about paint thinner or other similar solvent to get it cleaned off. (Testing it on a very small area to see, of course.) And then using a good spray polyurethane after.

It’s such a great piece! Good luck!

Might be easier to remove and replace them

Following... please post progress pics!