Wayne,
Voyageurs national park has a lot of rocks with paint on them near the portages. From the boats that run over them. Rocks are dark water is dark.. scrape.... Its a sign of lots of rentals, but it is also warning flag. My suggestion would be not only to layer on an extra coat, but to stich in an extra layer of fabric under the bow and along the keel. It will give your SOF skin some extra durability on the grinding.

PS you should connect up with John Monfoe. He has is trying something similar with a wooden yak and is filled with good ideas.

!RUSS