This might be a job for Captain Tolley's Creeping Crack Cure!
At least that's what it used to be called. It appears the Capt. dropped the "Creeping Crack Cure" part of the name. That's a shame. It's the only reason I remember it. Saw it in a marine / boat supply store once.
Captain Tolley'sThere's a short video about it at the bottom of the page.