Epoxy needs UV-protection. An opaque color is therefore the best option. But for most it would be unacceptable to hide the result of many hours' hard work under paint. Therefore, over to the next best alternative - varnish with UV-inhibitors.
I prefer a two-pot urethane coating, high gloss and tough and with a superb longevity - 10 to 15 years on a canoe, that in contrast to most boats often spends most of its time under a roof. Traditional oil-based varnish (Spar varnish) last approximately half that time. You can switch from two-pot to one pot at any time, but not the other way around, unless you remove all the old varnish. The strong solvents of the urethane coatings solve the old varnish base, resulting in a flat and grainy finish - irrespective of how many coats you apply.
To make sure the varnish of your choice has UV-inhibitors it is often enough to look at the price tag - if you are shocked it isprobably what you are looking for. A slightly more scientific way is to check with the manufacturer.
Three or four net coats is normally sufficient to protect the epoxy - that is four to five coats plus light sanding in between. One further advantage with some two-pot coatings is that they can be applied in short intervals without sanding. I have often done three coats during day one (morning, midday and afternoon finishing a couple of hours before dew), sanded lightly with 180 grit plus two coats on day two and completed the schedule with a light sanding with 220 grit and a finishing coat with a high quality brush on day three. Normally I do this outdoors on a calm day without too much pollen in the air. Insects may be a problem, but they are easy to remove with buffing. In a workshop it is not easy to avoid dust contaminating the surfaces - but careful cleaning and a little water sprinkled over walls and roof may work - perhaps supplemented by a plastic sheet stapled to the roof above the canoe.
The traditional schedule below may sound a bit ambitious and can be simplified, or of course be developed, after your own experience or perseverence. To follow it, though, results in a first class finish.
- Sand with 120-papper on cork or with oscillating sander.
- Sand with 180-papper until all scratches are out.
- Brush carefully clean and brush on a thin layer of varnish.
- Sand with 220-papper.
- Vacuum the surface and the floor.
- Rub the surface with a cloth dampened in alcohol.
- Rub the surface with a sticky cloth to remove dust. Spray a little water on the floor in order to settle dust and change to dust free clothing.
- Paint on next coat of varnish.
- Repeat step 4-8 according to your needs and patience. (With 2-components urethane one can often apply two or three coats between sanding - but sand always always before the last coat).
Congratulations! Go paddling.