Polyurethane adhesive is used for many years by the builders for foam core wing sheeting. These adhesives cure through cross linking with moisture from the air and the substrate. The bonds are extremely strong and temperature and water resistant. For this reason the adherence is unbeliveable, much better that using epoxy resins. In addition it is compatible with fiber glass reinforcement using epoxy resin, when you apply the PU adhesive when the resin is still wet. The problem is that it is difficult to find a reliable source of a good polyurethane adhesive, and some brand changed the composition over the years.
Gérard Prat who co-developped the use of the PU adhesive for wing construction, and also well know for having introduced (with Thierry Martinet) the vynil covering technic relayed the following information from Hervé Filliat who found a good polyurethane adhesive, compatible with our needs and constraints and providing about 1 hour of pot life (working time), which is long enough to sheet a 4 meters sailplane wing.
The reference if the following: Kleiberit Pur ADHESIVE 501-6
It is available in 500g or 5Kg packaging size. In France, this adhesive can be found here: FTFI
Thank you Gérard, and Hervé for this valuable information !