The scenario ....... hand rails attached to top deck, and through deck to
hand rails below.

The boat construction ...... fiberglass deck ( cabin roof ) , core balsa ,
fiberglass bottom ,, insulation ,, cabin liner.

If I want to follow Casey's book method: remove hand rails, drill out the
holes a bit larger, tape the bottom hole and fill with thickened epoxy,
redrill holes, reattach the hand rails ,,, the problem is the underside.
With the cabin liner to deal with, and the insulation, and all ......... it
is hard getting at the underside of the deck, cabin roof.

Would removing the hand rails, rebedding with 4200 , be worth while?

How can I get to the bottom of the bolt holes ?

If you have had to do this ,,,,,,,,,,,,, ????