Tar penetrates wood by flowing into surface pores, cracks, end grain, and rough fibres. Penetration depends on temperature, wood species, dryness, surface preparation, and how thick the tar is.
Tar does not soak into all wood equally. End grain can draw in much more material than a smooth planed face because the cut ends of the fibres act like open channels. Rough timber can also hold more tar than polished or resin-rich wood.
Temperature and viscosity matter. Warm, fluid tar can wet the surface and move into small spaces more easily than cold, thick tar. A very heavy coating may look strong but remain mostly on the surface, while a thinner coat can sometimes bond better because it reaches the outer fibres.
Wood condition matters just as much. Tar struggles on wet, dirty, greasy, painted, or sealed wood. It needs contact with the fibres. On old timber, weathered surfaces may absorb unevenly because some areas are open and dry while others are polished, contaminated, or already saturated with old coatings.
Penetration is part of why tar can be useful for outdoor timber. A coating that keys into the surface is harder to peel away than one that simply sits on top. The goal is not endless depth, but good contact with the working surface of the wood.