what i usually do is throw the old mold into the dish washer and clean it really good. then take a razor blade and dice it up into 1/8" pieces. then when i pour a new mold i take a disposable acid brush and paint the piece with the new silicon. as i pour the new silicon i liberally toss the diced pieces in there and make a loose layer then pour some new silicon in to cover and then toss some more diced pieces in. i can usually replace half of the new silicon with the old diced up mold and i've never had an integrity problem yet.

-Klingbeil-