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?