Tobby Litaker of Charleston, South Carolina, asks:
“What’s the best sealant to use when reinstalling windows?”
Don Casey replies:
If you are talking about plastic windows, do not use polysulfide sealants, because the solvents they contain will cause the plastic to harden and crack. The adhesion of polyurethane sealants is compromised by the outgassing of some plastics, so these likewise are a poor choice.
If your windows are sandwiched between frames, standard marine silicone is the sealant to use. Make sure that you do not squeeze out all of the sealant while it is wet. Essentially, you want to create a rubbery silicone gasket. Tighten the fasteners slightly after the silicone cures to put this gasket under compression.
If your windows are fastened directly to the fiberglass cabin side of your boat, consider using a structural glazing sealant, such as Dow Corning 795. Normally used to affix windows in office buildings, these silicone products have much stronger adhesive properties than marine silicone sealants. For step-by-step guidance on window replacement, you might take a look at the newest edition of my reference book, This Old Boat.