If you can find it, a product called Anchor Seal will allow it to dry without cracking. However, I'm not positive it will protect it out of doors. If I were leaving it out of doors, I'd thin down some Boiled Linseed Oil and apply it until it won't take any more. (the idea being to saturate it) Then when it refuses to take any more, I'd apply a full-strength coat of the Boiled Linseed Oil. Beyond that, I don't know what to tell you. There's not really a product designed to make your stump "completely" waterproof in the elements. Good luck though. I'm partial to stump bases myself.