If you have ever had a water leak on your ceiling leaving a brown mark, you’ll know how untidy it can look. Also if you’ve tried to redecorate just by covering it with several coats of Emulsion paint you’ll realize that is not an ideal solution to the problem.
Sadly there are very few water bourne products that will successfully trap and prevent a water mark bleeding through till finished coatings. Traditionally oil based undercoat would be used to seal the stain. There was also products such as Gulttonglass or Stipptick sealers which were faster drying, but unfortunately didnt work best with emulsions.
Nowadays there are various priority stain sealers in brush and spray applications which perform the task very well and enable you to successfully prevent the stains reappearing.
A word of warning ensure the source of the stain has been cured fully first as no stain blocker can actually hold back a persistent water leak