Best way of removing a paint stains ?

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my NEPHEW came home from nursery school on monday covered in paint. i sprayed the paint marks with vanish pre wash, left it to soak in for a while, then i washed them on a hot wash with biological powder. most of the paint came out, but the black paint remains.... i've washed them again, but the stains are still there. the paint is supposed to be water based poster paint, but i'm at a loss as to how toget rid of it. any ideas on how to get black paint stains out of fleecy cotton jogging trousers ???