November 4, 2015

Sorting out the urethanes

I’VE DONE MY FAIR SHARE of painting boats, both wooden and fiberglass, but I was only vaguely aware of the difference between polyester urethane and acrylic urethane. However, I was looking through some notes the other day and came across a copy of an e-mail that explains things quite nicely.

I believe I originally found it on the Cape Dory bulletin board. In any case, it was posted by someone called Brandon, of Fort Lauderdale, who owned a 1985 Cape Dory 25D called Seamona. Here is what he had to say:

“The difference between the two, without getting long and boring boils down to this:

“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 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 topside as we would with the polyester!

“OK . . . so maybe I got long and boring.”
Today’s Thought
Paintin’s not important. The important thing is keepin’ busy.
— Anna Mary Robertson Moses (“Grandma Moses”)

Mary has a cool, cool gown,
It’s almost slit to bits.
Who gives a damn for Mary’s lamb
When we can see her calf?

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