Check with Carla (65 Bowrider) - I think she and Skip used it in a lot of places on the bowrider they restored. Skip said it sanded great for a couple days but let it set more than that and it became rock hard! I think there might be better primer/sealers for use above the waterline.
I used it on the bottom of my boat which required lots of filler after I had it soda blasted, but based on Skip's advice, I only did one panel at a time, so I could sand the entire panel the next day before moving on to the next panel. I applied it with a roller and did a lot of sanding on it. (I'm not sure it's worth spraying if you're gonna sand it anyway.) I'm sure it's less than the recommended thickness for preventing blisters, but it's a trailer boat and I've never left it in the water more than 2 weeks. I just wanted something that would fill in a lot of the porosity that the soda blasting created in the gel coat. Besides that, the Signature Finish Paint I put over it is the only urethane I know of that holds up underwater, so I think it also provides some protection. Here's a slide show of the sanding/filling/sanding work I put together on that phase of the bottom painting project.
http://s188.photobucket.com/user/dkb...1051d.pbw.html
Here's the post I did a few years ago on the whole project:
http://www.classicseacraft.com/commu...inting+project