* * I have seen a wing that was yellowed on bottom and still quite
white on top. The owners had been using wx/block on top only. So, does
it work? I'd say "Yes".
*
additional note....
The owner of this sailplane thought the top of the wings sitting in
the old Komet trailer might suffer from UV.
And when assembled, the underside wouldn't have any UV issues.
The color difference was quite noticable.
GA