VARNISH AND LPU
If you have minor blemishes that you hope varnish will cover, stop right there. Varnish will not cover or fill anything. The shiny surface will make every ripple, scratch and sanding swirl more noticeable. If the surface is not smooth enough, fill with resin and sand again.
I have tried a handful of brands, but I've only achieved excellent cosmetic results with Z*Spar Captain's Varnish. I have also used Z*Spar Flagship but it leaves significantly more brush marks. Since Flagship offers more UV protection, one way to go would be to start with Flagship, wet sand smooth, and then top coat with Captain's.
I get the best results with a fresh can of Captain's using a foam brush. Old varnish never works as well, even if thinned. Hot weather is not good either. Varnish seems to work for me best at about 60 degrees and 60% humidity. Too much colder and the varnish tends to sag before it can dry. Too much warmer and spreading the varnish evenly becomes difficult.
System 3 water based LPU is a more durable coating than Captain's varnish, but it's harder to apply well. It's watery so it likes to drip and run. It absolutely requires cool temps and high humidity or it will show brush and overlap marks. And it never does get as smooth or as shiny as Captain's. However being tougher, it will likely look better after several weeks of use than Captain's will. After full cure it seems to take three times more effort to wet sand than regular varnish.
Unlike normal varnish types, LPU is non-yellowing.
It takes a lot less work to varnish a boat with LPU, mostly because sanding between coats is not necessary if done in a timely manner. Several coats can be put on in a day which saves a lot of time. However LPU requires more coats to get similar film thickness. I really appreciate the absence of obnoxious varnish fumes and being able to clean up with water.
If you like to mark areas that need sanding, do not use an alcohol based marker like a Sharpie. The marker ink will migrate all the way to the bottom of the finish. Similarly, you cannot wipe down the surface with alcohol. It will remove the finish. This problem goes away after several days.
Somewhat reluctantly my finish of choice for future projects is LPU.