That's the clincher really - I'm reading that a paint is technically more a plastic once it's dried - then surmising I shouldn't worry about it so much, and obviously nearly every product out there could be wiped clean enough to eat off, so if everything went well it would seem like a clinically applied measure of each layer going down without issue and the PC sheets only against a cured surface with no reaction possible - but condensation presents some microscopic level of problems arising, moisture being held against two material/substances could trigger something in the long term, possibly foreshortening an expected 15-25 year problem free roofing job to 5/< years. IDK.I guess any exterior paint of you colour choice then.... even fence preservative?
Condensation between the sheets and the board may be an issue to manage though.
Agreed and this too is on the cards - the original '80mm rigid foam composite' would have been an all-in-one product to that end, so I'll be fitting something to achieve the same insulation, the OSB and PC combined will give a thickness of 34mm - an intended thicker 35mm multiwall PC roof light may present a problem here as it will quickly take on heat - it's area is about 4x1.5m, might have to forsake that idea...So long as the osb is 'cold' ie if there is insulation underneath it there should not be an opportunity for condensation to form between the boards and the PC.
Thanks, worth expanding upon - I haven't been looking into corrugated materials, I've viewed them, but not regarding them in this context as any added insulation may have cavities, or as you describe the OSB backed on to them would have - I only really conceive of both a plastic inner & outer wall of a cavity as an inhospitable environment where problems are much less likely to occur - and the original box profile direction of the project was the only sort of product that is manufactured completely filled inside the corrugations.Xtian - I take it for reasons I can't immediately see, that you haven't considered Coroline or Onduline. Absolutely no bother to install, very long lasting and lays onto OSB without any further protection.