I have masked off and primed the area of the crack, and I'm really quite pleased with the result. Not absolutely smooth, but no part of the hull is. this is bent plywood, not moulded plastic. If you crouch down and peer up at reflected light, you can just about make out a slightly raised area where the fairing compound was applied, but looked at straight, it is invisible. I will try to rub down the vertical edges of the primer this evening with a little white spirit before it goes hard. That should feather the edge sufficiently for an overcoat to hide it.