make sure you scrape off or wire brush any loose bits mate ,
then use a fungicidal wash,
and when the walls are COMPLETELY dry,
i would give them a coat of stabilising solution (spirit based)
then a couple of coats of "pliolite" (oil based) masonary paint
this method will give you the best and longest lasting finsh
by the way dont do it till the summer
.
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however if your gonna sell the place just stick a coat of b&q special on and hope for the best