Here's what happened to the Urethane on my Flying Clown wing just sitting in the attic in extremes of heat and cold. The covering, scuffed up with OOO steel wool,was Doculam, a heavier grade of Phil C's SLC covering. It did adhere to the silkspan over balsa but I may have clear coated the fuselage with the old K&B clear epoxy.
The urethane was water based System Three Inc. marine paint over their two part primer. Nelson Products, who used to advertise in Model Aviation, distributed the products to modelers.
Ara