When decorating a surface which shows signs of water staining, it is essential that the cause of the leak is first located, repaired and the surface allowed to dry out, failure to seal the stain will result in its reappeared.  .  

Draw a faint pencil line around the outer perimeter of the stain, wait a few days and if the stain has grown the leak may still be live, alternatively use a Moisture Meter to gain a reading of the moisture content in a surface.

Seal stained areas with a trade quality Stain Blocking Paint, such as Zinsser BIN, Zinsser Cover Stain or International Stain Block or Polycell Stain Block, other brands are available in both liquid paint and aerosol.

It is important that the Stain Blocking Paint is undiluted and applied evenly over the stain and a few inches beyond the stains boundary.   

Allow the Stain Blocking Paint to fully dry before overpainting.