Putting a second coat on the bathroom ceiling? 
It does need it, if you look carefully in a good light but how often do you look carefully at the bathroom ceiling? It was painted because of some mildew staining caused by DS1's overlong showers . I've invested in a dehumidifier and I'm hoping that will stop it coming back. If you look carefully you can just see some of the marks through the fresh paint.
Do I have to?