3M 101
I used 101. Here's a blurb from their web site:"Seals between mechanically fastened joints on wood, fiberglass, metal and most plastics; above and below the waterline. Excellent resistance to chemicals, weathering, salt water, and stresses caused by joint movement." It lasts a long time in the tube, too. 4200 is an adhesive/sealant, at about 1/2 the strenght of 5200. It reacts with moisture to cure, which is why it doesn't last once the tube seal is punctured. It cures fast. For stanchions, I think 101 is the right stuff.See link.