I believe "Inducryl" (sp?) was (to quote the manufacturer) a "self vulcanizing rubber roofing compound". We first tried it on the Antonito cars years ago with great success. If I remember you spread the goop on the wood, put fabic (or canvas) over it and spread more goop on top of the canvas. As it was rubber, it didn't harden up like tar, it stayed flexible and water tight.
Unfortunately, the company went out of business. The owner died, no one wanted the company. I think his wife sold us the remaining stock about 10 years ago.
It was good stuff though....