Painting over mildewed windowsill / Knots

7 Dec 2013
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United Kingdom
My windowsills are black with mildew in places and I'd like to paint them white. I was wondering how to treat the mildew areas to stop the black coming through the paint? (Was going to undercoat/prime then two coats of satinwood) Have sanded but still quite black.

I'm also painting some new pine wood - skirting and door trim - and have treated the knots with one coat of Colseal knotting solution. My worry is that the knotting solution is quite dark (I realised after I'd bought and applied it you can buy white and clear ones....) and after putting on two coats of all-in-one undercoat/primer, I can still see the knotted areas showing a little. Was going to top with two coats of satinwood but stopping now incase I'm doing something wrong....

If anyone can advise me I'd be grateful....
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