Oh my Russell....my head is hurting on this one!!!
First off...ask yourself if you need fiberglass cloth at all!!!! Depending on the build, I am comfortable saying that glass on the sides is unnecessry. On the SlumberMAX (a ceder strip over ply build), I just got through applying 4 coats of rolled on epoxy on the sides, and plan to either varnish or clear coat for UV protection. On the top, I will use the Raka 4 oz x 60 inch plain weave cloth.
Raka Cloth On the roof, I figure I
might get some durability, I am not concerned about strength!!!
Secondly...if you decide to use cloth...don't use a woven mat, use a plain weave...the lighter the better for this application. 4 oz and 6 oz are common for canoes and kayaks. 3 oz would be ok for this application but it starts to get "tender" where you have to be extremely careful when handling it to prevent snags.
Thirdly...you should plan on having the substrate sanded and blocked, don't think you can use filler and a high build primer to get the surface you desire.
Hey....I like your vision of the end result!!! I hope I can help you get there!!!
