Stains appearing on plaster/paint - which sealant?

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Ive recently had a room skimmed, and have just put the first mistcoat on.

When the skim was done i noticed a line of dark "stains" across the ceiling.. I painted over them and within a day they are showing through the paint.
The only thing i can think of is this is remnants of a water leak i had a couple of years back from the radiator above, and i think the line is level with a joist so maybe some kind of mould.

I have seen mention of using some zinsser paint to cover these, but am unsure of which one is best suited for the job.. theres still at least one more coat of paint to go on the ceiling before the finishing coat so maybe one of the sprays would work best.

Edit: attached a pic - not great but should give an idea of what it looks like
 

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MisterG81, good evening.

Looks like residual damage from previous leak?

As noted along a joist or Plasterboard line?

Suggest you try any of the Proprietary "Stain Blocks" or a flat oil paint as a base then use Emulsion. It may take a couple of coats.

eddiewakey, Sorry but Gloss is not a recommended stain block cover, it is not designed to act as a blocker.

Ken.
 
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Does anyone have any recommendations to which stain block product to use?? "BIN", "Cover UP" "Cover stain" - all from the same company!!
 
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Does anyone have any recommendations to which stain block product to use?? "BIN", "Cover UP" "Cover stain" - all from the same company!!

Never honestly noticed a difference between any of the stain blocks I have used, if you have some oil based undercoat knocking about then just use that, you may have to add a bit of white spirits to it to aid flowing but it works a treat or even damp seal works very well.
 
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