What causes alligatoring in paint films? We was up to out eyeballs in alligators considering that very question just yesterday morning. I was at Middlebury College helping the maintenance crew understand how to "knit" exterior woodwork and paint materials into an effective weather envelope. Alligatoring usually begins with cracks in the thick paint film that is the result of heavy paint buildup. The first cracks usually appear perpendicular the length of the board. This is often noted in terms like "perpendicular to the 'grain of the wood'" although this first set of cracks does not have anything to do with the grain of the wood. With vegtible-oil binder paint, especially traditional linseed oil, the oil molecules are in the form of relatively long "strings." When the paint is brushed onto the surface, the brushing action tends to orient the stringy molecules in the same direction as the brush strokes, which is usually along the length of the board. When the paint dries, the oil molecules combine with some oxegen from the air, which kicks off polymerization, rapidly forming even longer molecules as the oil changes from a liquid to a solid. As the paint film ages over the decades and centuries, the oxidation continues at a very slow rate, and the long molecules break down into shorter strings that are shrinking slightly. Stress builds up in the paint film, there is more stress along the stringy molecules than across the molecules. Since "broken" molecules can no longer hold the paint film intact, cracks form, perpendicular to the orientation of the molecules--usually perpendicular to the length of the board. This first set of cracks may eventually penetrate down through the entire paint film to the wood beneath. Water gets into the cracks and then into the wood. The wood then expands and shrinks more with these greater changes in moisture content. This rate of expansion and shrinkage is much greater across of the grain of the wood than it is along the grain. So, the board is expanding and shringing across its width, which stresses the paint film forming cracks parallel with the grain of the wood. Now there are cracks in the paint file going across the board, and along the lenght of the board, forming little rectangular plates. With all these cracks much more water is getting into the wood beneath. The water wants to get out of the wood, but cannot pass through the extreme thickness of the heavy paint build up, so the water begins to push the paint off of the wood, especially beginning right near the crack in the paint film. Remember how the paint film is shrinking? Well it turns out that the later layers of alkyd resin paint shrink even more than the earlier layers of linseed oil, so the film is tending to curl, which helps pull the paint off the wood. The rectangular plates curling slightly around their edges completes the visual similarity to an aligator's skin. I can't recall if I learned about this from Bill Fiest or Steve Irwin. -- John by hammer and hand great works do stand by pen and thought best words are wrought www.HistoricHomeWorks.com -- To terminate puerile preservation prattling among pals and the uncoffee-ed, or to change your settings, go to: <http://listserv.icors.org/archives/bullamanka-pinheads.html>