Some areas are cored, some aren't
Sometimes, the only way to know is to drill the mounting holes. If you find wood core or worse, a space between the skins, seal with epoxy. Once water gets in there is no way to get it back out short of ripping up the deck. I wouldn't rely totally on 5200, especially since it will be exposed to sunlight and the seal can deteriorate. Besides, sealing with epoxy is fairly simple. OK, I'll admit it adds a bit more time to the project, but it's still faster and easier than trying to fix rot...PeterH23 "Raven"