Of the various brands I looked at last fall when shopping for satin
varnish,
only 1 or 2 recommended gloss varnish as the base coat[s]. HOWEVER, most
of
the satins are rated for interior use only. Maybe they aren't designed to
hold up under hard use, weather, or UV.

Epifanes has a UV resistant satin. I used it on my windowsills, looks good
but it's
obviously an interior application, so I can't tell you how well it holds up
outside.

Matt