Wood filler

[Tim Lamb]

Following the thread on mdf window boards vulnerability to water, I wonder if there is an explanation for the *bumps* in mine.

The securing screw head recesses were filled with polycell polyfilla and sanded to level. Prime, undercoat, topcoat in low VOC solvent based paint (Dulux trade brilliant white).

Now, two years later, the filler is noticeably proud of the mdf!

It is likely that a decorator attempted to wash off emulsion splash using water.

Mdf shrinkage? Corrosion of screw heads? Expansion of polyfilla?

I have fitted mdf pre-painted skirting and am about to cover up the screw holes. Any thoughts?

[Phil L]

The mdf has shrunk, causing the filler to now be proud of the surrounding timber

Skirtings are usually ok, it's the sun getting on the window boards that cause shrinkage

[harry]

There is a special primer for MDF. Has there been some movement in the MDF, ie it has been pushed down & the screws have pushed the filler out?

[Tim Lamb]

Oh. This was a wood primer.

Not visibly. There are no cracks at plaster joins. Sunlight shrinkage is doubtful as the North facing window is as bad as the others.

I wondered if polyfilla encourages corrosion in steel as I have regretted using a nail to block the spout on a PVA glue dispenser.

[Tim Lamb]

Also on North facing windows. The building is heated so the mdf could have dried out.

[polygonum]

And would you care to explain what effect that has upon subsequent shrinkage?

[harry]

MDF has always been pretty stable IME. Maybe it got damp before it was installed? I assume the screws were galvanised, they all seem to be these days, so no rust.

[Tim Lamb]

I can see I am going to have to chip one out to check.