By all reports starting with the MSDS and researching forward. Trade secret stuff but it acts like spar varnish mixed with drying oil and Japan dryers though some would say poly varnish. A mixture that had been around before Newell wrote his book. BTW, all BLO is low grade linseed? Where does the high grade stuff go, cheap barn paint? Depends on where you get it. Speaking of Newell he was no fan of paint store linseed. Writes you don't know how long it's been on the shelf. Wrote the stuff begins to degrade after 6 months IIRC.
Anything you buy marked "BLO" is sold in hardware stores and made of junk oils.
Linseed oil is great oil for finishing, but it is also edible and all the high grade stuff is used either for food, or pharmaceuticals, or other very demanding uses. Tung, for all practical purposes is identical, however many are allergic to the stuff and so it has no use in food and such, therefore the higher grades are available as finish.
