February 26, 2012

Two different varnishes

ONCE UPON A TIME I had a racing dinghy with a varnished foredeck that was much admired for its perfect glossiness.  I hadn't taken too much care with the application of the varnish, but when it had cured good and hard I rubbed it down with very fine waterproof sandpaper and then buffed it with a rag and metal polish.  It sparkled like liquid amber.

A few years later, having built a new dinghy, I bought a different brand of urethane varnish.  When it came to the finishing touches, I was very disappointed.  No matter how much I sanded and buffed, I simply couldn't get it to shine like the first one. Only recently did I realize why.  There are two different kinds of urethane paints and varnishes. There's polyester urethane, like Awlgrip and Perfection, and there's acrylic urethane, the kind of finish they put on new cars in Detroit.  One of these varnishes can be  polished to a high sheen.  The other can't.

I found this information on the Cape Dory bulletin board in a post by "Brandon" dated November 7, 2010.  Here's what he said:

"Polyester urethane molecules are much smaller than acrylic molecules. So when they cure, the polyester urethane forms a tighter matrix, which gives a harder, more abrasion-resistant film, with better chemical resistance than the acrylic.

"Acrylics are more forgiving in application, trap less dust and are buffable. When an acrylic urethane is buffed, due to the lower cross-link density the melting point of the resin is much lower, i.e. it is softer. When buffing is carried out, the resin-rich layer "melts" and reflows into the scratch. It is possible to retain an intact resin-rich layer at the surface protecting the pigments and not losing significant thickness. The edges of touch-ups can be blended carefully in the same way. Long-term performance is not affected, as much of the resin layer remains.

"With the polyester urethane, the paint is a very hard rigid film, and to get rid of a scratch you need to cut deeply into the paint, leading to the exposure of the pigments. This looks shiny to begin with but the long-term performance of the finish is now compromised.

"I am currently following the build of a 95-footer in Viareggio, Italy. We are using Awlcraft (Snow-white), and almost finished painting her. I am happy to use the acrylic because we have found fairing issues on the hull, even with the white paint, and with the acrylic we can re-fair this 6-square-foot area, reshoot the area and blend in. We don't have to repaint the entire 95-foot hull side as we would with the polyester!"

Considering how tricky it can be to apply the better-known twin-pack polyester urethanes by brush, it might be more practical to use the acrylic urethane, without worrying too much about the finish, and then fine-sand and buff your way to a perfect shine, as I did with the first dinghy.  

It's good to have this knowledge, but sometimes I wonder how much longer it's going to take me to learn everything I really need to know about sailing and maintaining small boats.

Today's Thought
Knowledge and timber shouldn't be much used till they are seasoned.
— O. W. Holmes, The Autocrat of the Breakfast Table

 "How's your glassblower friend?"
"Not so good. He inhaled by mistake and had to go to the doctor with a pane in his stomach."

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

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