Great question — answering as the UphillTech team:
Naphtha (e.g., charcoal lighter fluid) is solvent that can dissolve pine tar (which readily drips from branches onto grass or benches below the tree), without harming most fabrics (test in an area hidden inside, first). Caution! naphtha is flammable. Solvent-based cleaners, such as Lestoil , should also be effective -- with the same caution on flammability and checking for possible damage to fabric.
Full answer by DrMoishe Pippik: https://lifehacks.stackexchange.com/a/26586 (CC BY-SA 4.0).
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