Minwax and rustoleum both have an exterior Spar Varnish in a rattle can. I've used the minwax polyurethane quite a bit and have been pretty happy with the results on new cabinets and furniture I've done.
I was looking at a sample of birch and walnut that had dark walnut danish oil on them and I really liked it. Can the Spar varnish be applied over the danish oil as extra protection? Or would that not work?