Help - can't get Blackboard clean!

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Does anyone have a good tip for cleaning a blackboard properly? It's just got chalk on it (but from some time ago) but rubbing it hard with water won't get it off. I'd be grateful for any ideas!

baby wipes. they're good for everything!

Would fairly liquid do it?

Jif would definitely but may also ruin the blackboard!

2nd the babywipes. But if all else fails, repaint it.

B+Q sell blackboard paint

Thanks everyone. Will attack with babywipes and otherwise repaint!