hard, slick, abrasion resistant surface for the bottom of canoes
I did a bunch of rather crude tests on epoxy/graphite mixes, comparing them to plain resin a few years ago and as far as I could tell, they were neither harder or more abrasion resistant than plain resin. In fact, they were noticeably softer and less abrasion resistant. The advantage they had was in slickness. For example, your canoe might slide off of a river rock easier with the graphite coating, but the material itself tended to scratch deeper than plain resin did and left more stuff on the rock. The test method used was admittedly not very scientific, but was an attempt to simulate real world activities. Sections of cedar planks were covered with the two materials, weighted, and literally pulled across a big brownstone rock that my wife dragged home from Lake Superior. Using the same weight and about the same speed on both planks, the result was considerably more black stuff (graphite mix) left on the rock than white stuff (plain epoxy) though it took a bit less force to move the graphite plank. This was using about 10% graphite in the mix, which is what Gougeon Brothers claim is enough to form a good UV barrier for the resin (which is a plus for graphite mixes as UV is always a concern when working with epoxy). Higher amounts of graphite in the mix tended to make it even softer, so there was no advantage to them. The addition of some cabosil (think powdered glass) to the epoxy and graphite would harden the mix, though at some point you're going to change the viscosity enough that it might get difficult to spread it evenly. Using the hammer test on hardened chunks (put them on the bench and hit them with a hammer) there was a difference in the brittleness between plain resin and graphite mixes and for what it's worth, the more graphite added, the less brittle the sample was and the better it would survive.
My personal conclusions were that in most cases, I don't think it's worth it as a coating, since most of us don't tend to pound our wooden canoes recklessly down shallow rocky streams. Cosmetically, since epoxy doesn't self-level like paint (even when rolled and tipped) if a graphite mix is just applied and left that way, it's pretty ugly. On the other hand, when sanded smooth and left with a satin finish from fine-grit wet-sanding, it is a pretty darned elegant charcoal gray. Sanding it is a pretty messy job though with all that black dust. If I was going to do a graphite bottom, I would also do it mid-summer and "bake" it. If you apply the resin and then roll it out in the sun, or stick it under a makeshift dark-colored tarp-tent on a hot day so that the temperature it is curing at is substantially higher than normal, it will raise the heat damage threshold of the finished product.
I didn't bother to do a similar test with 407 or 410 fillers. I've used and sanded enough of them repairing and rebuilding boats to be pretty sure that the same drag test would have worn down quickly to bare wood and the hammer test would have powdered the impact spot. The aluminum flake "Barrier Coat Additive" seemed to make the toughest sample in terms of abrasion. It wore down even less than the plain resin and was pretty tough stuff without seeming brittle. Aluminum canoes are known to "stick" on rocks, but I don't believe there was enough present to worry about that. It's also a bit harder when you go to sand it smooth, but not too bad. I have, on occasion, seen paint problems with it though. When you sand an aluminum/epoxy layer smooth, you are exposing tiny bits of raw aluminum. These start to oxidize immediately as all aluminum does. Anybody who has ever tried giving a Grumman a home-paint job is probably aware that paint doesn't stick very well to aluminum unless you do a lot of prep first with etching and/or special primers. I have seen paint jobs on epoxy/aluminum flake mixes where the whole surface developed very tiny (pin-point-sized) fisheyes (holes in the paint layer) where the paint wouldn't take on the flakes. In one case on a two color boat, one color did this and the other color of the same brand of paint went on fine. The aluminum powder does make for a tough surface, and also a more water-resistant epoxy layer. Epoxy is the best stuff we have available at resisting water penetration long-term, but it's still not perfect. When you fill it with aluminum flakes, the water would actually need to zig-zag around the flakes to get through - thus the reason it is used for moisture barrier coating. On a canoe as a canvas filler, it might be worth trying if you have a primer that would handle the potential paint problem. Left raw, it's a pretty ugly, battleship gray color.
