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Re: polycarbonate window thickness?
andy, 3M makes special adhesive for polycarbonate, 3M 4200 as I recall. West Marine actually STOCKS it, along with their own version, and oddly enough the 3M product is also way cheaper than the WM version.
If you do not use an adhesive that specifically is designed for polycarbonate, it WILL let go within the year.
All the plastics do suffer from thermal expansion, so if it is "short" now and the temps are 60F, I wouldn't worry, I'd let the sealant fill it. But if temps are 90F and it is short...it is only going to get shorter, so remind them "Measure once cut twice." [sic]
Kinda depends on how long the plastic is, and how big the gap is. You can look up the expansion specs online for whatever brand and grade of poly it is, it can easily be 1/16" per foot for the range of freezing to heatstroke.