Re - Could you use it on the outside of a Balsa wood hull
I don't think it would be sandable Martin, as it initially dries a bit soft and rubbery. Might be ok for lining the inside of a hull perhaps, very slimy and spreadable when wet. Slightly heavy, will go quite firm after a few months and harder as it ages, (a bit like chewing gum
does) I've lined my galv bucket with it (which was getting a bit rusty from mixing cement) to see how well it stands up. I've also spread it on a 4x2 board outside to see how it holds up to the sun and rain. It could be quite useful as a ballast as I mentioned as it would also strengthen the hull once eventually set, (you would probably have to resin seal the hull first to prevent the petrol eating any glue)
The goop remaining on my tug hull (around 45") went quite firm after a few months and made the hull really tough (only that and some 8oz cloth for the center 1/3 of the hull. Had no problem with it going out of shape. Worth a play to see what else you can do with it. I did a test with two 3"x 12"x3/4 pieces of dressed pine where I just glued about 3" on 1 end of each, let them tack off then clamped them overnight. took about 30lbs of pull to separate them by pulling on the non glued ends.
Probably as good as commercial contact glue. Not sure how long it lasts ( my tug was still good after 5 yrs), but it's almost free, and might be useful if you run out of the proper stuff. I know you love re cycling Martin!