You don’t necessarily need to repaint the drywall over water stains. There’s a much easier method to get rid of those unsightly yellow or brown water stains on ceilings. This includes old water stains on any drywall surface - both ceilings and walls.
This solution is as plain as day but is also extremely effective. Just apply a mixture of bleach and water carefully onto the stain, and it will go away, looking like it has never been there to begin with.
In order to remove the stain, combine 1 cup of bleach and 3 cups of water in a spray bottle. Then simply spray the bleach solution onto the stained surface. We recommend using protective gloves and glasses. Let the bleach mixture sit for 24 hours, soaking in any drips with a bucket (to avoid bleaching the flooring). The bleach will fade the stain, but it will also eradicate any mold deep in the plaster.
Note that this only works with light-colored or white walls and ceilings, as it may bleach darker pigments, and you'll need to paint over it.
Before you decide to remove the stain, make sure that the root cause of the stain has been dealt with; that there is no more leakage and that the wall is 100% dry, otherwise, the brown mark may return. Water stains can appear for a variety of reasons: a damaged roof, a leak in the upstairs bathroom, or a leaky boiler are all common culprits.
Last but not least, note that if the stain led to cracks or chips in the plaster, you will have to repair those as well using a repair kit that’s available at most home repair stores. In most cases, all you’ll need to do is scrape away any loose bits and fill in any cracks with plaster putty.