Traditional sailcloth treatments use tar, oil, or related dressings to protect natural fibres, but they are best approached as heritage craft finishes rather than recipes for modern working sails.
Old sailcloth treatments were part of a world of natural fibres, wooden vessels, tarred rope, and constant maintenance. Tar-based dressings could darken cloth, reduce wetting, and help protect fabric in hard conditions. The result was practical but heavy, smelly, and very different from modern synthetic sail materials.
For craft or display work, use heavy natural cloth and make a small test panel. A thin dressing is easier to control than a thick coating. Work the treatment evenly into the weave and keep the cloth stretched or flat while it settles so it does not glue itself into unwanted folds.
Modern sails need specialist care and should not be treated with craft tar mixtures unless the maker of the sail or material specifically supports it. Tarred sailcloth is best for museum-style interpretation, historical props, model work, decorative covers, and educational samples.