Step forward Ronseal "Wet Rot Wood Hardener" although other makes are available. It flows like water and appears to be cellulose based. The side benefit is, if something goes wrong in your covering, it doesn't go completely hard for a few days and in the meantime an extra application will allow you to lift and reposition.
Having completed the covering by fastening the edges, I heat shrank the fabric and then painted over all the wood areas with the fixative. Unfortunately a couple of bubbles appeared, but with another quick blow over with the heat gun, they disappeared.
I think I may make this stuff my go to preference for covering and stuff the Postman who works against me. I will survive
