I have newly (well a year ago) cast sills in concrete I now wish to paint
white. The rest of the house has white gloss painted sills. What system of
paints and preparation do you recommend?

I have used International door step paint before which was excellent but a
bit pricey and I'll need quite a bit. It seemed quite thin and soaked more
into the concrete giving a feel that the interface was more secure. I've
got others to put top coats on, I guess it is a case of rub down and apply
the same top coat you recommend.

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Cheers,
Mike W