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

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