Actually, most satin varnishes dont have the UV resistant properties that the glossy varnishes have. Main reason to coat with glossy first...
From Epifanes: " Gloss finishes are more weather
resistant and offer better protection against UV and weather
. Building up with 3-4 coats of this before the Rubbed Effect will maximize protection. Secondly, the build-up of gloss gives the overall appearance of the finish more depth
and clarity. Epifanes does manufacture a version of Rubbed Effect called Woodfinish Matte. This is one of the only “Rubbed Effect” finishes containing U.V. filters. This addition makes it very suitable for exterior use."