Tar works best on dry wood because it can contact the fibres, spread more evenly, and protect the surface without trapping existing moisture beneath the finish.
Dry wood is the normal starting point for a controlled tar finish. When the surface is dry, tar can wet the grain, enter small checks, and bond to the material rather than sitting on water, frost, or damp dirt. The finish also becomes more predictable in colour and texture.
Brush or rub the tar into the wood thinly. End grain, knots, cracks, and exposed edges often absorb more than smooth faces. If the wood is very dry and porous, it may darken quickly and take more finish than expected. A test patch helps judge how much tar is needed.
Dry wood still needs good construction. Tar cannot compensate for joints that hold water, feet that sit permanently in damp soil, or surfaces that cannot dry after rain. It works best when the object already sheds water well and the finish adds another layer of protection.