My tinning removal technique was given to me by Mal Paso, who made quite the informative thread on creation of Patina on brass.
https://castboolits.gunloads.com/sho...-Patina-Recipe
I'm not sure if it's mentioned in the thread or not, but probably is.
Heat the mold to casting temp (about 375-400º), I use a hot plate. Using beeswax and a stick (I use chop sticks), touch the area of tinning with pure beeswax so a little bit melts, then rub with stick. This is challenging to do while mold is hot, and it cools off fairly quickly, so re-heating happens multiple times, until tinning is gone. I mount the mold blocks backwards on handle, so I can access mold faces easier. Have a folded up cotton cloth on bench so you don't damage one mold face, while working on other mold face. THEN...the fun part is cleaning off the beeswax after all tinning is removed. clean as much as possible with heat and cotton cloth and q-tips, then I use MS to clean what remains, then dish soap, Hot tap water, toothbrush to clean the remaining MS residue.