Epoxy/graphite/silica bottom


LOVES Wooden Canoes
I'm finishing up a 16' Prospector that will be used occasionally for whitewater. It will have a shoe keel and a graphite/epoxy/silica layer below the waterline. Then I'll paint over the black layer with 2-part white epoxy paint. Any problems with getting epoxy paint to adhere to a graphite/epoxy base?

I'm out of the colloidal silica that I've used in the past to add to the graphite/epoxy mix, and wondering if I can't just use the cheap silica sand sold for sweeping into the cracks of landscape pavers.
Hi Gary,

Good to see you here.

You sure are trying different things these days. :)

But this one, I have no experience using the graphite, other then seeing it on other folks canoes, but to be usefull, wouldn't the graghite have to be exposed to the rocks? It would seem that covering it up would be defeating the purpose of the grapgite. Again, no experience here, just thinking out loud.

I have used a graphite epoxy layer (for abrasion resistance), followed by the recommended primer, and a colour coat of paint. I have not had any adhesion problems.
There are some epoxy fillers that have certain paint adhesion problems, but I haven't had any with graphite powder as long as it's cleaned of any blush and properly sanded first. If I didn't have any colloidal silica on hand, I'd just use a graphite/epoxy mixture. It will be a bit softer than the same with a bit of colloidal silica added to harden it, but it''s still pretty tough stuff. The sand may also be made of silica, but it's in a very different form and I don't think it will do the same job. I'd be worried that I might be creating a finish that sticks on river rocks and ledges like crazy - when I'm actually trying to coat the boat with something slippery. It might also be a real bear to sand smooth and get ready for paint.

Dan, you are correct. If painted over, the graphite will do nothing until a rock scrapes through the paint, exposing it. At that point it can, in some cases, help the canoe slide off a rock. Whether or not it slides off with less damage than plain varnished epoxy filler coats would have sustained is a whole different debate. Most people do leave the graphite surface unfinished (either just as rolled on, or sanded smooth) to cut down on paint repairs. It tends to look a little lumpy if you don't finish sand it, but once sanded it has a pretty nice charcoal satin look to it and is pretty easy to patch when needed. It's kind of a messy sanding job because of the black dust. The graphite content blocks UV at the surface, so a varnish topcoat isn't needed as a UV shield.
I had graphite on the bottom of my canoe and it was significantly more difficult to remove than the regular epoxy. It absorbed a lot of heat from the heatgun w/o allowing the epoxy to soften.

It is my understanding that aside from imparting hardness, the graphite also acts as a lubricant and as such is often added to the bottom of racing boats. It is certainly slicker than the plain epoxy, it also continually sheds graphite, running your hand across it leaves your hand gray with graphite as if you'd just written a several thousand word essay with a 4B pencil. I have my suspicions that any paint would adhere well to it, but I don't know. I did hit a few rocks one dry season and there was no surface damage, but when I removed the glass the other day, you can clearly see the "bruises" in the cedar. Can't say what would have happened w/o the graphite though.

I've never heard of a keel on a white water boat, I thought maneuverability in white water precluded the use of keels, not so?

Good luck.