I am not a big fan of 4200/5200 as sealants. They are terrific adhesives but do not have much flexibility with cured. For anything that has motion Sikaflex, Rule or other flexible sealant is better. I also use butyl in a lot of places. The stuff NEVER cures and stays gooey. The only knock is that it may tend to ooze out for a while until things stabilize but it does clean up easily with mineral spirits. Another thing that works really well in some cases is neoprene sheet (see McMaster-Carr) I don't think I would use if for the stanchion bases since the flex loads could be pretty high but when I replaced my wooden handrails a few years to I made individual gaskets under each foot and fastened them from below with lag screws. No leaks and makes it easy to remove them to take home for refinish.