Phil Addison wrote in message
If you want a good sealer for the concrete sill have a look at
Bondaglass G4 eg:

http://www.arghamvillage.co.uk/products/details/180.html

That's an interesting one - a pond sealer. Should be waterproof then!
Not sure about the colour though, Browny translucent, Mid Green, or
Black. Could over paint it I suppose. Thanks for the pointer.

I think a fibreglass resin would be better. G4 cures on contact with
moisture which may not be a good thing if the concrete is damp close to the
surface. Foul stuff to work with too.