I've been researching Tung Oil of late for a different purpose, but wanted to let you know that the chinese have used tung oil to water-proof wooden boats/ships for many hundreds of years. In pure form it takes a very long time to dry, so the new hot thing is to use citrus solvent as a 1:1 thinner with pure tung oil (China's is reputed to be the best quality), get it in there, and layer it in.
It hardens not by eveporation, but by oxidation, so a layer goes on, wait 4 hrs, smooth in whatevers still on the surface (or it can be a potential problem), and wait the remainder of 24hrs to add the next coat.
This finish is easy on the liver, easier on the lungs, smells nice while you work and you can use it on kitchen countertops and cutting boards safely.
I'd guess that for weather/waterproofing, you'd want quite a few layers.
You can also add more later without undoing everything you accomplished in the first job. Just a good cleaning.
Hey, and it doesn't mildew the way linseed is famed.
Good luck... anyone find a great price on gallons in the U.S. let me know.