I don't use any rollers or brushes and getting a smooth surface isn't that hard.

I apply epoxy by pouring it on, rubbing it into the fiberglass with my gloved fingers and then squeeging the excess off. Once I'm done I go back to the beginning and apply another coat, wait several more hours and apply another. Each one is applied by pouring it on and squeeging it off.

Once the applied coats have cured I sand it, but make sure not to go deep enough to sand into the glass. Then I apply another thin coat to make it look good until varnishing.

The squeegee method will get it pretty smooth and will allow you to apply a paper thin coat.

: Looking for some advice. I've spent days sanding and recoating my guillemot
: trying to achieve a perfectly smooth/fair finish before applying varnish.
: It seems whichever technique I use (foam roller or foamie brush) I end up
: with a surface that isn't mirror flat... it seems the epoxy goes on too
: thick even with my best efforts to apply it sparingly. So now my
: questions: 1) how smooth must the epoxy be prior to varnishing and is
: their a trick to getting the viscosity to help to this end?

This one is hard to answer, because how do I explain smoothness? Obviously the smoother the better.

: 2) does varnish need to go onto a mirror like finished surface - or can it be
: the final saving grace to achieving a perfectly gleaming hull?

Again, the smoother the better, If you have a bumpy surface it will show. Then you will have to sand the varnish to get it smoother and recoat.

: 3) will I do any catostrophic UV damage if I go for a week long paddle
: without having applied any varnish? (assuming we get some nice weather!)

No, many suggest paddling the kayak for a month or so before applying varnish.