you dont need to seal the seam with playdough. after you brush in your beauty coats in both halfs, put em together and then brush a thick layer into the seam. let that cure and then you can pour in your latex. i do this and i never have a leak. because my seam lines are some of the worst. if you have a huge gap in the seam where the mold cracked out, you can use a piece of cured latex to plug it up. i had to do that in a mold that was missing a big triangular piece, before i finally fixed the hole.
EDIT:
although, playdough in the seam might not be a bad idea. you might have to barely trim the seam if you get the playdough in there good.
if you do this, let us know how it goes.