Folks
The varnish on my hull has done well to last this long I reckon but I think I need a more durable solution.

Another solution which has been discussed (and tested- search online) is Interlux VC Performance Epoxy which is a 2-part hull finish containing a Teflon-type additive.
It's white, but can be tinted to grey with (you guessed it) black iron oxide.
I used it on a kayak project last year, but that boat ('rolling kayak') has only seen a few trips to the beach and the pool, so I can't vouch for its toughness.

It's pretty easy stuff to work with.
Expensive, but if you have to buy epoxy (vs using 'leftover epoxy') for an alternative treatment the price won't seem so steep.