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How to get water stains off a ceiling
Find the source of the moisture.
You may find an obvious leak in a pipe or fixture on the floor above the damaged ceiling, but it may also be more difficult to pinpoint.
If you don't locate the source of the moisture prior to doing the repair, the problem will just keep coming back.
Wear gloves, safety glasses and a dust mask while looking for and repairing the leak. Depending on how long the leak has gone unrepaired, there could be mold present.