You have done exactly as others would have done.
UniBond is PVA adhesive and when dliuted acts as a sealer that prevents the staining bleeding into the top surface of the paint, which can then be seen.
A professional will take a view on whether a ceiling can be quickly sealed in this way, but is unlikely to engage in sanding processes that you get to get a smooth finish - it is far quicker (and hence cheaper when time = money) to bring a plasterer in, to reskim a poor surface finish.



