To mount electronics, I built what is basically a hollow rectangular surf board and painted it with System Three water based linear polyurethane paint. As you can see in the second picture, after 7 years gloss retention is still quite good, and seems to rival solvent based paints and blows the doors off single part formulations I have used. This piece is exposed to maximum sun 365 days a years.
The material is fairly easy to work with, but more finicky than solvent based LPU paints like Awlgrip. It has to be applied in the shade and because there are no solvents other than water, you can't overwork it and still get it to level. For this piece I slopped the paint on with a foam roller, then tipped it briskly once with a foam brush and let it settle. It should be applied thick so there is sufficient body to level before it gels. Done correctly, you can't tell it isn't sprayed.
All things considered, this looks like an outstanding candidate for top side paint jobs, including decks.
WR-LPU Topcoat - System Three Resins, Inc.