The gentlest way to remove the tinning seems to be with a hot mold use a contractors pencil or piece of wood and push it along the surface of the mold and the lead comes off. If the mold is cold you can very carefully scrape it off with a razor blade.
I learned about the heat cycling to form an oxide after I'd used my brass molds enough to build up an oxide layer from casting. I'd imaginge baking the mold in the oven at 500F for an hour or so ought to do the trick.
