To tar a boat hull in a traditional context, apply a suitable marine or pine tar coating only to compatible wooden construction, after the hull is sound, clean, dry, and properly repaired.
Tar has a long association with boats, but it is not a universal fix for every hull. It belongs mainly to wooden and traditional craft where the coating is part of a maintainable surface system. Modern GRP, metal, rubber, plastic, and painted marine systems usually need different products.
A hull must be structurally sound before tar is applied. Tar can help shed water and protect exposed timber, but it cannot cure rot, close moving seams permanently, replace caulking, or make unsafe construction safe. Clean away loose coatings and let the timber dry to the condition required for the chosen product.
Brush tar thinly and work it into seams, end grain, rails, and exposed timber surfaces without leaving heavy ridges. Marine exposure is severe because sun, salt, abrasion, and movement all attack the coating. A tarred hull needs regular inspection, especially after launching, hauling out, and long wet periods.