canvas after fiberglass

Rod Tait (Orca Boats)

I have an old canoe in my shop that was built by a small company from the prairies in Canada (no longer around) The boat had fiberglass on it which the customer wanted to replace with canvass. Did that just over one year ago, (two summers paddling) and now the canvass is pretty well rotten and falling off. I have other boats with same canvass, sealer, paint ,etc that are over ten years old with no problems. My theory is that on the hull with planking, tight seams and all sealed with epoxy, the canvass can not breathe and dry out after use. Lots of moisture in behind the canvass. The canvass literally fell apart in my hands. Any thoughts and should I advise to just put glass back on with better epoxy than the polyester that was on there. We don't know if it originally had canvass, but there seemed to be evidence of old canvass when I originally took off the fiberglass but not 100%sure. I don't want to see the boat back every year or so.
Same product I always use. Robson mastic latex filler. I have this product on many boats in the past ten years without any problem. I do not know how this boat is stored, but it is in Whistler all year round and I suspect left outside to the elements by the damage to all the rib tops that it had first time round in my shop, like it was left on the ground upside down for a long time. There was just a lot of moisture trapped in behind the canvas when I pulled it off this time. I am leaning towards re-doing the fiberglass.
The canvas and canvas filler I use do not have any preservatives in them to prevent rot and mildew. I apply the clear liquid wood preservative [ used on treated lumber, also available in a green liquid ] to the canvas after the canvas is on the canoe but prior to the filler. Horrible stuff, use a respirator. Allow it to dry overnight. This treatment also will shrink the canvas which is an added bonus.