I have the same problem before, when I was painting on panels that I prepare myself (MDF panels primed with acrylic gesso) and varnish after finished.
But when I change the surface to a canvas panels the varnish dries faster in a few hours.
That make me think the surface has a lot to do with how the varnish dries. (BTW I use Winsor & Newton Dammar Varnish)
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