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hvsds-
The problem with a modern material like Toupret (or cement) is that it has a very different porosity (I guessing almost zero) compared with sandstone / limestone.
This can mean that water gathers in unexpected places, and can damage the remaining stone though dissolving / freeze-thaw, or other water action.
NotThamesWater
What should I use to smooth out this concrete windowsill? Or is it not worth the hassle and just paint over? Borrowing some ladders next weekend which I hope to not fall off and sort this out and repaint them all.