Tar usually affects softwoods by soaking more readily into open or porous areas, darkening the surface, and giving weather-exposed timber a more water-repellent coating.
Softwoods such as pine, spruce, and fir are common candidates for tar because they are widely used in fences, sheds, cladding, boats, and outdoor structures. They are often easier for tar to wet and colour than dense hardwoods, especially when rough-sawn or weathered.
The absorption can be uneven. End grain, knots, splits, and rough fibres may take much more tar than smooth boards. This produces natural variation in colour and sheen. A single coat may look rustic rather than uniform, which is often part of the traditional appearance.
Tar helps softwoods by reducing surface wetting and adding a sacrificial weathering layer. Softwoods still move with humidity and temperature, so flexibility matters. A tar coating that remains slightly elastic can be a good match for that movement.
Softwood tar protection is strongest when the timber is dry enough to accept the coating and the exposed ends, joints, and lower edges are not forgotten. Those are the places where water often enters first.