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I've sewn up sails with dental floss before now, really strong and hard-wearing - but they were only in the sun for a few hours a day for a few weeks a year - beware of sunlight on any manmade fibre unless it's choc-full of UV inhibitor.Any heavy thread used by professionals for sewing shoes, saddles etc will be OK - ask your local cobbler! If they use natural fibres its probably waxed.Cheers. PS. on a vaguely related subject I've just found a delivery note for my last polytunnel cover - it was 2013, and it's only got a single 50cm crack where the sharpest fold was when it arrived (also the sunniest - I should have put that fold on the shady side) - all the other softer folds look good for another 5 years. No problem at all anywhere it rubs on the frame (but my frames are nice cool plywood arcs on top of straight 4x2s - nice to know I got that right).The secret is buying plastic covers designed for latitudes like 35deg. - the decent stuff they have to use there has at least twice as much inhibitor than UK rubbish made for 50deg. - I don't want to advertise anything so if you're interested just google "almeria polytunnel covers" and dig down on any UK polytunnel firms that show up - at least one should specify the origin of their source.