If I understand correctly you are asking what to use as a structural gap filler to be used in compression at the base of the mast. If that's the question then the answer is NO. No caulk is structural in compression. Adhesive caulk like 5200 could be termed "structural" for gluing but not structural in compression where the surface isn't supposed to flex. Build the gap with structural epoxy filler. Bill P. **************Planning your summer road trip? Check out AOL Travel Guides. (http://travel.aol.com/travel-guide/united-states?ncid=aoltrv00030000000016)