I thought the only reason to put it in the sun was if the paint was not getting hard.
Well, it speeds it up, in cooler weather that can be a big help.
Also, after priming, it can cure it up faster so there's less shrinkage,
it makes a big difference down the road, like after painted and being in the summer sun a couple months,
a lot of times a really slick finishes need re buffing, but if you cure that primer a little extra it can
really lessen that, and the sun does that better than anything.
I like my primed panels to sit in the sun 2 days before sanding,
and my cleared panels 2 days before buffing.
I have noticed a much better hold out of the finish doing this.