Can you shed a light onto what went wrong

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Hello, I’m a painter in Australia and have used Builders Bog successfully many times for timber repairs prior to painting but I have a problem that I haven’t come across previously. The job is painting a new treated pine picket fence – the pickets are fixed with galvanised metal screws to metal shs rails. I have recessed the screw heads below the surface, filled with bog, sanded smooth, primed with oil based and then two coats weathershield.

After leaving the job for only a few days due to rainy weather I returned to find the filled areas have ‘divoted’ or appeared as sunk below the timber surface. The paint coating was intact but the previous smooth surface was now pockmarked where the screw heads were.

I have now refilled, resanded and repainted and am hoping for no repeats.

Can you shed a light onto what went wrong and how to prevent it? Is it the case of the timber expanding around the filler due to rain? Or maybe the metal rails heating up in the sun and transferring heat to the screws and shrinking the filler a bit?

Regards, David
23.2.2016

Hello David,

We see this in fine furniture – piano, guitar and coffin work where it is necessary to stabilise the timber at the humidity it will be constructed at for a week or more.Further, the screws must have clearance holes driven in the outer member and pilot holes in the threaded member. Otherwise what happens is the Builder’s Bog fill is flushed off by sanding, and then the ambient humidity rises, the wood swells around the inert filler and the carpenter swears the Bog has shrunk.

The movement is exacerbated by the compressed fibres around screw heads driven straight into the timber without pilot holes – as you would do on a fence batten. Obviously the piano procedure cannot be done for an outside fence and you are limited in appearance by the natural expansion and contraction of timbers as the climate changes.

This is not normally noticed or criticized for a fence and the only solution I see is to use pickets like fibre cement board and steel posts if it is not acceptable. By doing it twice you may have minimised or eliminated the problem. I hope this helps

Regards
Stuart Jordan
Managing Director
Chemical Specialties Ltd.
23.2.2016