Suitable goop / filler for bodging a windowsill in an uneven opening?

[google]

Hi,

I have a small window which has never had a sill as such - just a flattish plastered bottom to the opening. I now want to tidy it up a bit, so I've bo ught a length of ready-made MDF window-sill and trimmed it to fit.

With the sill in place, it becomes very apparent that the bottom of the ope ning is not level in either direction - it slopes down towards the inside o f the room, and also has a slight slope from right to left. I made a reason ably tight fit when trimming to length (more by luck than judgement, especi ally since the ends are not square either!) so it sits straight and level b y friction with the sides, but there's a gap underneath it of around 10-15m m (varies) across the front, sloping to nothing at one of the back corners and very little at the other.

I imagine there's a proper craftsman's way of fixing this with precision jo inery, but to be honest I can't really be arsed :-). Instead I have visions of spreading some sort of mortar or filler across the whole opening, then pressing the sill down onto it and tapping it down to level a bit like layi ng a big tile. Then smoothing off the front where it will squeeze out of th e gap, to leave a "finished" surface (or as finished as a 15mm strip undern eath a window-sill needs to be).

So, finally, to my question - what would be a suitable material for this, p referably obtainable in a chain shed tomorrow? If it also sticks the board in place then so much the better.

Cheers,

Pete

[Roger Mills]

I would level it with bonding plaster, and then fix the cill in place with gripfill once the plaster has set. I wouldn't put MDF onto wet plaster, and I wouldn't use it at all anywhere where it's likely to get wet such as a kitchen or bathroom.

The Gripfill may mean that the cill stands slightly above the plaster with a very small gap along the front edge. If so, finish that with a thin bead of decorators caulk once the Gripfill has gone off.

[Dave Liquorice]

Or select suitable length brass/stainless screws to use as adjustable packers where required. From the description thats probably three corners and extra along the front edge, if there is much give along that length. No holes filled or otherwise on the top surface.

Folded/rolled newspaper blocker in the gap along the front edge. Fill with plaster or polyfilla.

I'd avoid any serious fixing of the cill down you may want to remove it at some point. Stick the packer screw heads on a blob of grip fill or similar.

[Vir Campestris]

Our bedroom window cill has interesting bumps in it. I suspect condensation over the years has affected the MDF...

Andy