Window sill replacement and plaster repairs around window frame


New Member
I have replaced a external box sash window sill, and I am in the final stages of making everything good again.

One of the final tasks is to make the plaster good at the base of the window surround before attaching the internal nosing that runs horizontally at the base of the window frame.

The nosing sits relatively high up, so it does not cover the gap between the base of the window sill and the plaster.
I therefore have to extend the plaster up a little bit so that it covers the gap between the old plaster and the internal base of the window sill.

How should I do this to avoid future problems and hairline cracks etc? The plaster will partially sit on top of the wooden window sill before it merges with the old plaster.

I understand there are many options eg one-coat plasters, fillers, fiberglass mesh for reinforcement etc. Can someone please recommend a suitable procedure and products that can be used to get a durable finish? Does the job require particular surface preparations etc? Would it be better to use a mesh or membrane so as to not to bond to the wooden window sill in case this has movements etc?

Your input will be appreciated.



Well-Known Member
Seal well in gap with Pva or something. Toupret filler but get it well in gap. Mask sill and dado before you start. I use plastic scrapers 3 for a pound at wilko and cut to width to fill tight gaps like that


Private Member
Alternatively, get a length of 16 mm Scotia or whatever size suites, cut to length and return each end then fix to the underside of the cill. No need for filler, sanding or caulk, no chance of a air line crack appearing and, it's what a descent architect/ builder would of specified back in the day.