i have a house that is still happens to on the odd occasion, ive cleaned it for over 5 years now :

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it doesnt only happen on white paint either
i have a rather big house that they have painted the windows in some " hand-made paint" no 2 colours are the same, its 200 quid a tin aswell :

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its that type of house, anyways, i have 4 windows that get the sun all day that its ALLWAYS happens to, the lady has come to accept it now, we did cure the problem for a while she had her painter to coat them windows in a matt varnish, but of course the enxt time its painted it happens again, oh i should have added, they paint the windows every few months :

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, the painter works 3 days a week all year round at this 1 house, shocking waste of money