[VAC] Re: urethane v. oil based finishes

[Dr. Gerald N. Johnson, electrical engineer]

Tung oil and linseed oil are the most common varieties of drying oil.
Good tung oil is pure, no solvents and no urethane.

Many oil paints and varnishes are based on linseed oil. I see in my
Mechanical Engineer's handbook say that they also can include tung oil.

It says: "Spar varnish is of long oil length (lots of oil in proportion
to resin), made usually with phenolic or modified-phenolic resins, tung
or dehydrated castor oil, and linseed oil. Other spar varnishes are of
the alkyd and urethane types. Spar varnishes dry to a medium-hard glossy
film that is resistant to water, actinic rays of the sun, and moderate
concentrations of chemicals."

The resins and modifiers affect the appearance of the finish, whether
gloss, semigloss or flat.

I know that tung and linseed oil are "drying" oils that harden on
exposure to air. They do that so well that rags containing them will
heat hot enough to ignite if piled up in a corner.

I just finished a table made out of a Philippine mahogany door with two
coats of linseed oil. I rubbed it in, let it dry a bit, then rubbed off
the excess. Let it set a few days and did it again. My can of pure tung
oil (been here for decades, sold by Ace Hardware, made by Hope chemical
company) suggests the same technique. It did take a couple days or
longer for the linseed oil odor to disappear.

I don't know what is in Watco oil. But I know how to look on the
internet. The MSDS doesn't admit anything but solvents. There are many
colors available hinting at different stain contents. One web page
suggested mixing thinner, linseed oil, tung oil, and darkening it with
asphalt for the color. Other Watco pages said it can take many days to
cure enough to be dry enough to cover with varnish.

Oil finishes penetrate, and leave little on the surface.

My dad was born in a house that was never painted though the clapboards
were varnished with linseed oil. That house is gone now.

Gerald J.



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