Following recent discussion of mast flotation, I decided to insert a masthead pool noodle during a mid-season repair this week.
When I popped off the halyard sheaves, the absurdity of "waterproofing" mast rivets became apparent. At least on my Dwyer spar, the top of the mast is open to the air--or, if capsized, the water. The casting that contains the sheaves is not closed off from the mast extrusion.
Or it wasn't. I cut and shaped a thin layer of flat semi-rigid fibreglass board left over from another project and stuck it into the casting with silicone. Then I put new silicone into the rivet holes when I replaced the casting on top of the pool noodle. It occurred to me that if I had successfully sealed the spar, the noodle might be redundant extra weight at the top of the mast where I least want it. I guess I find out the next time the boat is on its side in the water.