Tar affects hardwoods by darkening the surface and adding water resistance, but dense or oily hardwoods may absorb it less evenly than softwoods.
Hardwoods vary so much that there is no single behaviour. Oak, ash, beech, elm, chestnut, and tropical hardwoods all have different densities, pores, oils, and natural durability. Tar will not behave the same on all of them.
On open-grained hardwoods, tar can settle into the pores and produce a deep dark surface. On dense, smooth, or oily woods, it may sit more on top and be slower to bond. Existing finishes, waxes, and natural oils can reduce adhesion.
The protective value depends on need. Some hardwoods are naturally durable outdoors, while others decay quickly if damp. Tar can add a water-shedding layer, but it does not override the wood’s biology, construction details, or exposure.
The main visible effect is usually colour. Tar strongly alters hardwood appearance, especially on pale species. It should be treated as a dark historical or working finish, not a clear protective dressing.