If the existing coats are in halfway decent shape, you shouldn't need to
sand much.
I've had GREAT success with Epiphanes varnish, but it is expensive. Plan on
a half-dozen coats or more if you go to bare wood, and a minimum of 3
otherwise. I like to give a couple days drying time between coats.
I wouldn't waste my time on anything that isn't a UV-stabilized spar varnish
if your boat is stored outside. Though I am dying to try a new product from
Epiphanes that doesn't require sanding between coats - if you try this,
please let us know the results.
I've recently been experimenting (on spare wood from my new hatch) with a
base coat of Gluvit epoxy sealant but i haven't overcoated it with enough
coats of varnish to know what the final results will be. Early results seem
to indicate that the sections of the wood I sealed are going to build up
smooth varnish coats much faster than the bare sections. Others have told me
that any grain sealer is useful in this respect, though I'm using the gluvit
for its water-sealing properties than for its effects on the finish coats.
My old hatch delaminated real bad, so I'm doing everything I can think of to
prevent this and make my new hatch last much longer.
There's no shortcuts to a good varnish coat - you gotta thin it
appropriately (50% with the first coat or two of Epiphanes on bare wood,
then 40%, etc... with the proper thinner) so it penetrates deep. Don't sand
in the same room you'll varnish in or you'll get bubbles all over
everything. In fact, don't walk in the same room :) don't go too thick on
the coats or you'll get runs. Don't look at the varnish in a funny way or
you'll get bubbles :)
Its a lot of work, but the results look so nice. Once you put a real good
thick base coat of varnish on this season, a light sanding and a maintenance
coat every year should be all it needs.
Don Casey's "this old boat" has a great chapter on varnishing. So does the
book "Brightwork". Both are published by International Marine/McGraw Hill.
"This Old Boat" is worth buying - its $25 or so from amazon or barnes and
noble online.