Update/report.
I splashed out and bought a new lot of epoxy sealer primer which Simon and I will share. Truly horrendous price for something we'll use so little of.

My fingerboard was definitely the worst of the two. I mixed up 20cm3, and without thinking too hard gave it three (very tiny brush) brushed coats. This was much too much since I had definite brush strokes on the finish, and so I had to sand scrape and goodness knows what, and by the time I got it satisfactorily flat some of the board had turned decidedly purplish. On the other hand all the loosish fibres were thoroughly encapsulated and I was happy with the mechanical integrity. Simon and I took a look, and decided to give it a go with lemon oil, and to my relief the colour turned acceptable, and I'm content with the result.

Simon's fingerboard is much better. I've given this a single wiped on and wiped off coat. Simon hasn't seen it yet, but I think its close to good, although lemon oil ill be an obvious thing to do.