Tarred timber can be recoated when the old finish is dry, bonded, and clean enough to accept a new thin coat, especially where weather has exposed pale or dry wood.
Recoating is part of the normal life of tarred timber. Wood tar and pine tar finishes are not usually one-time permanent coatings. They weather down, lose gloss, fade, crack, and thin on exposed edges. A fresh coat restores colour and water resistance before the timber is left bare.
Before recoating, brush away dirt, loose tar, algae, and dust. Look for wet pockets, lifted edges, and softened timber. A new coat should be applied to a sound surface, not used to seal in decay. Areas that face sun and rain usually need attention before sheltered undersides do.
Apply less than seems necessary at first. Old tarred timber can become overloaded after repeated heavy coats, leading to tackiness, black buildup, and cracking. A thin maintenance coat worked into dry, exposed areas is usually more useful than a blanket layer over everything.