Many car waxes advertise UV protection. You could put this on over varnish (give plenty of curing), or put on temporary until you get to varnish (wet sand before applying the varnish)

> A freind was showing me his CLC he built last year, varnished
> and then paddled all last summer (multiple trips). It was stored inside
> when not in use.

> When he pulled off the hatches you could see a distinct ring
> under the hatch rim. The rest of the deck had some cloudy discolouration
> despite 4-5 coats of varnish.