Leaky boards

Turpentine or other paint solvent will cut it.

Shellac will seal them

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Mineral spirits and a rag will remove the tar and not harm the wood. Shellac is the chosen one for sealing this kind of leakage but I wouldn't expect it to hold up against a lot of sappy wood over years of time. Several coats of shellac might do it. I might cut out sappy sections or use the boards as a "secondary wood" where leakage is not a big deal (garage/closet shelving, birdhouses, construction, etc) . Expect your cutting tools will need a good cleaning after cutting through sap pockets, for that, kerosene. Or, save all the extra work, consider buying clear boards.

Agreed. Turpentine is the natural solvent for pine rosin. And shellac is the basis for Killz and Zinnser's blocking type primers.

Joe