Tar can make wood more water-repellent, but it is better understood as a traditional protective coating than a perfect waterproof barrier. It works by slowing water entry and helping exposed timber shed rain.
To use tar for water resistance, choose suitable bare timber and prepare it so the tar can reach the surface fibres. Dryness matters. If the wood is damp when coated, moisture can be trapped behind the finish and the protection will be weaker. Clean, open-grained exterior wood is usually a better candidate than sealed, painted, or polished wood.
Brush tar into the grain, cracks, joints, and end grain. These are the places where water enters most easily. Thin, even coverage is more useful than a heavy glossy layer that may skin over, crack, or stay sticky. If the object has feet, legs, or ground-contact areas, tar can slow wetting but cannot remove the need for drainage and air movement.
Tarred wood still needs inspection. Water can enter through fastener holes, open joints, untreated undersides, and later cracks. A tar finish is most reliable when it is renewed as part of ordinary maintenance rather than treated as a permanent one-time waterproofing job.