'The Painters are in' is an affectionate term for that unfortunate and disgusting time of the month when the female species can bleed for three days and not die. To 'pay the painters', then, is the act of replacing one cunt rag for another. Cunt rag, of course, being a tampon. This usually costs about 50 English pence.
'Where's Cheryl gone? She was here a minute ago.'
'She's just gone to pay the painters'
'Oh! I didn't realise she was having the place decorated.'