Every now and then someone comes into my shop, and in the course of conversation volunteers to me that his (it’s always a him) family had a secret formula for a finish that had been passed down for generations. Of course he wasn’t going to share it with me because then it wouldn’t be a secret anymore.
So I would have to tell him what it was: 1/3 boiled linseed oil, 1/3 spar varnish (I never understood why it had to be spar varnish and not simply any varnish), and 1/3 turpentine. Now we might use mineral spirits (paint thinner) instead of turpentine, but this was an ancient formula, surely before there was mineral spirits.
Surprise on his face. How did I know?
Adding varnish into linseed oil is a way to make the linseed oil a little more protective and durable – not much, but a little. Doing this goes way back, at least well back into the 19th century.
And it is the basic formula used by Watco Danish Oil and other brands. There is perhaps no other finish about which so much mythology has been created: it protects the wood from the inside; polymerizes in the wood not on the wood; makes the wood 25% harder (really?); or this one from Minwax, which owned Watco Danish Oil at the time – bleedback leading to glossy spots at the tops of open-pored wood is caused by micro-organisms in the wood!
This last one is pretty wild, but I recently heard what I think is the wildest one yet. The smaller molecule oil in the mixture penetrates deep in the wood to give it a rich look while the larger molecule varnish stays on top to provide sheen. Think about that for a minute.
I’ve come to the conclusion that a lot of people write about finishes without really understanding them. But they are clever, and they know just enough to be able to make up stuff that seems correct within their understanding.
The problem is that they speak with authority because their explanations are published, either in print or on the web. So others begin repeating these explanations, and soon they show up published elsewhere and become part of the mythology.
— Bob Flexner