To tar a wooden fence, use a suitable wood tar or pine tar on clean, dry timber, work it into the grain with a brush, and let the coating settle before the fence is exposed to heavy handling.
A tarred wooden fence is usually treated for weather resistance, colour, and a traditional dark finish. The best surface is bare or previously tarred timber that is dry, brushed clean, and free from loose paint, mud, algae, and flaking old coatings. Tar belongs on the wood itself, not on a layer of dirt that will come away later.
Apply the tar thinly and work with the grain, paying extra attention to end grain, cracks, joints, rails, post tops, and the lower parts of boards where water tends to sit. Pine tar and Stockholm tar can be thick, so warm weather and a soft brush make the job easier. If a product is intended to be thinned, use the thinner recommended for that product rather than guessing from unrelated recipes.
A fence does not need to look glossy and wet to be protected. Heavy coats can stay sticky, attract dust, run down boards, and collect at the bottom edge. A restrained first coat followed by inspection is often better than one overloaded coat. Recoat when the timber begins to look dry, grey, or exposed rather than waiting until weather has reached bare wood.