Hemp rope can be preserved with tar by coating the fibres lightly so they absorb less water and resist outdoor decay better, while still remaining flexible enough for the intended craft use.
Hemp rope was historically valuable because it was strong, useful, and vulnerable to wet conditions. Tar helps by adding a dark protective coating around the fibres. It reduces how quickly water soaks in, helps bind loose fuzz, and gives the rope a longer working life in damp outdoor or maritime settings.
Use dry rope and apply tar in small amounts. Work along the lay of the rope so the coating reaches the outer strands. If the rope becomes glossy, dripping, or clumped, too much has been used. A preserved rope should look darkened and treated, but it should still bend and handle in a controlled way.
Tarred hemp rope is not clean rope. It can smell strongly, mark hands, and leave stains. That makes it excellent for historical display, garden lashings, rustic pulls, and outdoor craft, but unsuitable for decorative interiors where pale walls, curtains, or upholstery will touch it.