Sealing a stain on plasterboard before repainting

My daughter recently had a Mansard loft extension, it has an en suite which has a fan in the (flat) ceiling. The ceiling was painted with emulsion.
After the recent rain there is a water stain all around the fan. She hopes to find a roofer that can fix the leak as soon as possible.
I seem to remember that you must seal such a stain before you can re-emulsion it.
Can you tell me a suitable sealant to use, and can it be done before the the roof has been fixed?

Cheap oil undercoat is good.

You should get the roof fixed and allow the stain to dry out before painting.

Dec

My daughter is letting the property, the tenants move in on Saturday.
The whole place looks like a shining new pin except for the stain.
We hope to get a roofer to look at it in the next couple of days.
I plan to visit tomorrow, idea was to repaint the ceiling!

Unfortunately it is not a perfect world!