I'll see if I can locate some and try it. Some things take real looking. Not necessarily safer, different, sure to be costly. Never had any cause to use it. That's why I'd like a source for real boiled linseed.
Real spar means a natural resin oil varnish.
It simply was the makers most wear and weather resistant varnish. Not always the best choice. Then you have seen what I was talking about. Primarily in industrial arts manuals for school shop. There is always more to know.