[Palifox]

Back in 1997 exterior seating was built outside a building in Canberra, Australia. It was built onto a hip high concrete block wall which ran within a few degrees of North-South. The seat was on the eastern side and a line of trees sheltered it from the western sun in the afternoons.

The seat was hardwood timber, perhaps 45mm thick, 120mm wide, really solid stuff, supported by steel brackets underneath. The brackets were fixed to the timber by heavy screws, from below, these did not reach the top surface of the timber. The brackets were just visible from a distance if you looked hard enough. From close up they were not visible except through the gaps between the timber, perhaps 10 mm wide.  There was no visible sag in the timber between brackets.

The timber was varnished, probably polyurethane. After a few year's weathering, the varnish began to flake and erode. I noticed that the greatest erosion was in the areas between the brackets and the least erosion was immediately above the brackets.

Why did the varnish show least erosion above the brackets?

The seating has since been demolished as the building is being extended, or I might have been able to supply a photo. This has baffled be for some time. One of life's minor mysteries? Or is there a real explanation familiar to folk in the paint business?
Palifox

[Borek]

Am I right in my thinking that what you are basically asking for is why the paint was the weakest at the points of the highest deflection? Even if the timber was very thick it should work a little under pressure. The harder the coating, the more it will be prone to breaking when bent.

[Palifox]

I have asked this question on other forums and your suggestion of the cause being related to the deflection of the timber is the best I have seen so far. 

Thanks