I don't claim to know anything!
But I am building a modified "night heron double" in a sof style. I'm using 8oz poly from Dyson, and a coating called "Hi-pec sun barrier". I used it on an airplane I built several years ago and it was fantastic. It seals the fabric, is u.v. stable, impervious to any chemicals(even paint stripper),and also adhears to the frame. I will probably not even sew the seams, as on the airplane I just overlapped and glued and it looked real good and smooth. Final finish will be a two part polyurethane. May also use "bed liner" on the keel strip as suggested in a past post. Just to be different! Hope it works out! :)