calfater

A specialized nautical/shipbuilding verb meaning to caulk a wooden hull — driving oakum and tar/pitch into the seams between planks to make them watertight. Essentially never used outside boatbuilding and maritime history.

• Les charpentiers de marine calfataient la coque avant la mise à l'eau. — The shipwrights caulked the hull before launching it.
• Il faut calfater régulièrement un bateau en bois. — A wooden boat needs to be caulked regularly.
• Le vieux pêcheur calfatait sa barque chaque printemps. — The old fisherman caulked his boat every spring.

This is a specific maritime technique (sealing hull seams), not a general word for 'to seal' or 'to waterproof' — for a window or door, French uses calfeutrer instead.

Often confused with calfeutrer.