It will leak and allow too much movement. Stanchions need hard surfaces in actual contact to prevent movement, and the sealant
is just to fill gaps. Hairline cracks and seams will allow water
Also -- some people allow sealant
to set before tightening down the screws/bolts, then tighten some more. I don't agree with that approach because it breaks the sealant bond surrounding the screws/bolt heads and threads. The bond is important, so my approach is:
1) clean hardware
and surfaces in solvent to eliminate oils and contaminants from the maker's machining of the hardware
2) Apply sealant and tighten down hardware, but not too tight -- over-tightening fasteners in fiberglass
can cause internal crushing and create weak points.
3) You're done.