Among my boatbuilder acquaintances, there is not unanimity about varnish,
but we've narrowed it down to a few brands which are OK. The leading few
are Epifanes, Z-Spar, The West Marine house brands (Z-spar re-packaged), and
Interlux, but there have been a couple of positive mentions from otherwise
respectable people, of Le Tonkinois. Myself, I use Epifanes mostly. It
goes on a bit thicker and therefore builds faster. I have no experience
with LeTonkinois.
Whether it's going onto teak, mahogany or spruce makes no difference.