B. Comegys wrote:

Hi All ;
I have just finished repairing the gelcoat on my fiberglass sea-kayak and
am looking for a clear-gloss varnish to cover it with. I did some
searching and came up with a couple of ideas, Interlux Super Spar 90 or
Z-Spar. I looked up Z-spar page and it said "not to be used below the
water line", so do I need to worry about this? What is your preference?

Why would you varnish a fiberglass boat?

Gelcoat is meant to be the outermost surface on fiberglass boats and
function as a sacrificial layer. You can buff it to a high gloss, just
like a factory finish. I don't know of a single production boat that
uses any kind of finish over the gelcoat. Varnishing would give the hull
a yellowish cast and make repairing the finish much more difficult,
without providing any benefits.

Varnish is used on wood boats to seal the wood. It's used on
wood/fiberglass boats to protect the epoxy from UV exposure. Neither of
these is necessary on a fiberglass boat with gelcoat. I strongly suggest
that you don't apply anything over your gelcoat.