The first coat could be thinned but spraying is not normally recommended with urethane. And why are you asking about the first coat in specific?
I have done a lot of boat work in my life as well as lived and traveled on my own boat and I agree 100% with Steve about Epifanes. That is all I use.
There is a 2 component urethane for outdoors but last time I checked, several years ago, it was about $150/qt. Great stuff and is very temperature sensitive for application.
As for MinWax Spar Varnish, it is a joke. It shouldn't even be allowed to be sold with a name like Spar Varnish. I doubt it would last more than 2 months in direct sunlight during the summer in the south.
Tony B Retired woodworker, among other things.
"Strive for excellence and settle for completion" Tony B