The art/act of varnishing seems to be enveloped in mystery. The more people you ask for the best way to do it, the more answers you get. Frustrating!
After I had consulted rec.boats.building, dk.fritid.sejlads, Epifanes, Hempel, One Ocean Kayaks, Kayak Shop and some forgotten sources, I boiled it all down to do it this way:
I wanted the hardest possible surface, hence I settled for Epifanes PU Clear Gloss Varnish.
The compound I had the best result from, and found easiest to apply, was:
Varnish / Hardener / Thinner = 100g / 47g / 7g
The -5% hardener gives a bit wider wet zone.
The max 5% thinner makes it easier to avoid runners, and it is faster to build up thickness.
I found it easiest to use a foam brush.
As preparation you need to wetsand the epoxy with apr. 180 grain (120-220). Much finer will give a poor surface for bonding, and to coarse will show thru the varnish.
Between 6-24H second layer is applied. (optional, you may want to add more layers)
After 12-18H I wetsanded with 220 grain. (manufacturs recommends between 220 and 320)
Final layer. (there is no grain to fill with varnish, that is long done by the epoxy)
Recommended procedure for maintenance:
Wetsand with 180-220.
Wetsand with 220-320.
Varnish (no later than 24H (48?) later, the varnish needs to bond into previous layer)
First sanding must not be finer than 320, that it is not course enough to ensure bonding on fully hardened varnish.
All times are with temperatures around 16-22°C
Many will nag about the few layers, lack of sanding between layers, course grain, but: It is for a kayak that soon will get its fair share of scratches from everyday use. On top of that it will most of the time be covered in salt blotchs.