On my 1965 and 1975 the original thru hulls where pipe nipples threaded into the glass. A proper thru hull will have a skin fitting and a valve with a flange that is bolted to the hull. The thru hull is the skin fitting. The valve is the sea cock. DeBond breaks down 5200 and most caulks that are not silcone based, leave any sicone at home. It has no use on a boat.
Check out spartan hardware from Robinhood marine or Blake's secocks. For proper fittings that will last 50+ years.
Our center board pennant pipe was also threaded and glassed. We sourced a long bronze nipple and re attached with west system.
Carl Damm
DAMSELv
1976 #593
Stuart Fl
