: When I built my gaskets on my kayak I used graphite powder from a used
: LazerJet cartridge. . It
: worked fine and was no cost. Check around for a used cartridge.

Graphite is to laser toner as diamonds are to coal, or carbon fibers are to pencil leads. There are a lot of forms of carbon materials. They are not identical.

The materials used in a laser printer are mostly plastic based and melt when heated. This is what fuses them onto the paper. While there may be some graphite in the mix, I'd guess that they probably use other forms of carbon to color the toners. Graphite, like diamonds, is harder and more expensive than lampblack or soot, which are common sources of black coloring pigments.

I'm sure you got a good dark color from the toner, in fact, it may be far superior to graphite as a colorant -- but I doubt that it would give the other structural attributes that graphite is known for.

People who use graphite on hulls are looking for something that will toughen the resin and provide a more slippery hull. Supposedly a mix of epoxy resin and graphite reduces water friction. I think you would have to do some sanding to the finish, and expose some of the graphite crystals to get this effect. this should leave a dark gray finish. I don't think you'll get the reduced resistance effect at all from toner, but the color should be a darker black. I'm not sure how much toner will toughen the epoxy film.