Water stains have a way of coming back again and again, even after the cause of the problem is fixed and the water has dried. Often tannins in construction lumber are carried by the water through the drywall and are deposited on the surface as the water dries. Tannins can be difficult to seal and if there is a high mineral content in the water it’s worse.
These are two good examples of how the tannins radiate to the edge of the stain as the water dries.
The key is to use the right sealer. The “big gun” is shellac. You would be hard pressed to find a stain this won’t take care of.
Many water stains can also be addressed with various other sealers such as Kilz. Once the stain has been sealed you only need to paint over it with a quality topcoat.
Water stains always have a root cause. Whether it is a build up of condensation in the attic, leaking roof or a plumbing issue, it is important to investigate the matter and correct the problem.