Having very little patience and a friend who was an autobody man, we thinned out the varnish a bit and sprayed on all the coats in one day with no sanding at all. Finish was good on three boats we did that way and no problems. I would want someone better on the paint gun than myself but it was fast and took far less varnish than we estimated or than the building instructions had estimated. If possible that's the way I would go (will be doing three more that way when the weather warms). (it did leave the shop a sticky mess but that's another story.)
good luck
don