Spar Polyurethane . . .
I'd use 'em. If you're worried about moisture getting into the glue joints, you could brush another 1 or 2 coats of exterior (Spar) Polyurehtane. It'll bond well to the existing polyurethane. Spar provides a bit more moisture resistance and has some additives for UV protection.
If you want to be sure it's polyurethane and not possibly shelleck, put a drop of alcohol on the finish. If it's shelleck, the alcohol will disolve into the finish. If it's poly - it won't.
"If you're doing all the dos, you ain't got time to do the don'ts"