Internal window sill - best way to fix.

[aleatory]

MDF window sills suffered some water damage. The suspect: sealant on outside of window perished, fixed now.

Is sanding & filller (not sure which?), followed by a primer (zinser bullseye 123) & paint the appropriate action?

 

[phatboy]

That's a window board, sills are for outside

It looks stuffed to me, I would remove it and replace. B&Q do that style of board but quite a bit wider so you would need to chop it lengthways.

Or replace it with some proper wood then paint or stain.

When I replaced mine, I stuck them with Sticks Like ****, and put a couple of concrete blocks on top over night (Cardboard between so the concrete didn't damage the new boards primer.

 

[Gazman16]

As above ^^
Or you could cap over them with PVC http://www.eurocell.co.uk/window-finishing/window-boards
The PVC ones do look very good when fitted well (whichever way you go your going to have some fun fiddly cuts to do) but they do scratch easily, If your going to have ornaments or plants on there I would recommend putting something under them to prevent scratches.

 

[aleatory]

How do you get them out without causing damage to the windows / walls?

sill: seems like it works for both inside & outside... http://www.collinsdictionary.com/dictionary/english/windowsill

Wood: agreed - strong preference for proper wood over MDF or PVC (not a fan of a plastic feel).

That's a window board, sills are for outside

It looks stuffed to me, I would remove it and replace. B&Q do that style of board but quite a bit wider so you would need to chop it lengthways.

Or replace it with some proper wood then paint or stain.

When I replaced mine, I stuck them with Sticks Like ****, and put a couple of concrete blocks on top over night (Cardboard between so the concrete didn't damage the new boards primer.

 

[phatboy]

The dictionary is incorrect

To remove the old I would use a multitool, or a big chisel down the middle and lift out. There may be screws holding it down, but I expect the filler would be quite visible now as the woods all swollen up. You would have to try hard to damage the windows, they won't be attached to them at all, or at worst some caulk or silicone which will just pull away.