Good wash down with sugar soap, thorough rinse, light sand for a key then one or two coats of Zinsser BIN.
Zinsser BIN is magic stuff and sticks absolutely solid to this type of surface, dries dead quick so you need to work fairly quickly and it can be painted over within the hour. Plenty of ventilation as its pretty volatile and make sure you have methylated spirit handy for clean up as it doesn't recognise white spirit.
You shouldn't need an undercoat unless there is a lot of colour showing through the BIN - couple of coats of OB eggshell, jobs a good 'un!
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