Pine tar and turpentine are mixed only when a thinner, more brushable traditional finish is needed, and the mixture should be tested in small amounts because turpentine changes drying, smell, and flammability.
Turpentine can make thick pine tar easier to brush and help it wet rough timber more quickly. It is used in some traditional finishing mixtures, especially where cold or heavy tar would otherwise sit on the surface. The purpose is workability, not making tar harmless or suitable for every material.
Use a small test batch and add thinner gradually rather than creating a large unknown mixture. Brush the test onto the same wood that will be used in the project and watch how it darkens, spreads, smells, and dries. If it stays tacky, becomes patchy, or soaks unevenly, the mixture or surface preparation needs adjusting.
Turpentine is a strong solvent and must be treated with respect. Work with ventilation, keep it away from flame and sparks, protect surrounding surfaces, and store containers properly. A turpentine-thinned tar finish is best for exterior timber and heritage craft, not indoor decorative objects that need a clean smell.