Interior polyurethane will degrade very quickly under sunlight, it must have UV blockers in the coating and may well come off very quickly as its not flexible enough. If I put this kind of coating on exterior timber, it would fail within 6 months.


For cheap silicone sealant, go to a ceramic tiling supply centre or a uPVC supplier-they supply to the trade at much lower prices than retail. I pay a £1/ tube for my work supply but can pay £10 for the same product if I buy from a diy outlet.


As a professional decorator, I am not aware of anything that will outperform thinned silicone sealant in this use especially as its so cheap. Even the very best alkyd/acrylic exterior coatings are not as flexible or tough as silicone sealant.


A good elastomeric sealant (Sikaflex) has good potential for use on fabric but is more expensive than silicone and is untested in a thinned form by me. Its much tougher than silicone and may well last longer. Could be well worth experimenting with.