as ian points out spartite is not a sealant it is a chocking system, although correctly installed it usually seals the mast at the collar but only at installation time after the mast has been out once the spartite collar join needs some proper sealing. [sealant + boot etc]
as for the internal seal, the best solution is an internal dam pushed up from the bottom when the mast is out, and then when the mast is in, pour sealant on to the dam via a hole in the mastwall. i have used Duram 195 successfully many times. Dam should obviously be above the partners, so the water can drain onto the deck.
Good luck.