In the past I've used an epoxy produce, available locally, with carborundum in it. It is a bit like putting on very thick treacle but it does work. To apply, a small container of water and a well wetted finger melding the lumps along the keel. The area to be covered should be masked off and the rest of the hull covered with a plastic sheet. It is also a good idea to have a sheet on the floor too. All of this to stop leaving a white layer all over everything looking like a bad paint job.
Others use straight epoxy and mix in carborundum and it should be easier to apply. A builder of one of my designs I was talking to recently was going to use epoxy and fine sand from the local beach.
No matter what you use it will wear and need replacement eventually, that is acceptable especially when the kayak is given a hard life running on to gravel beaches. All part of general maintenance.
What have other builders used and how do they find applying it and life span? What mix, epoxy to carborundum ratio?
Silicon Carbide or stone dust
I have used Silicon Carbide, mixed in to the last fill coat at 10% by weight, which is amazingly good for abrasion resistance, but is a dirty grey colour (the Silicon Carbide is actually a fine, black powder, but the mix is grey).
Alternatively stone dust is nearly as good, and only gives a very slight haze to the epoxy. I also mix this at 10% by weight.
Both improve the abrasion (scratch) resistance immensely. And when the scratches get too bad just re-coat.
For any boats which are intended for rack gardening I also put a layer of kevlar on the football, it is a bit pricey and heavy, but close to indestructible.
Justin