Damp spot in new plaster. How to manage

[Sef1928172]

Hi all. Looking for advice in how to handle a damp spot in new plaster skim. Solid brick walls

This spot has not fully dried out. When it rains or is humid in the room, the patch gets bigger (second pic). There is no external indication of water being able to get in here but previously there looks to have been some holes filled in with filler internally that correspond almost exactly to where this spot is, so I wonder if that has something to do with it.

Do I need to hack off plaster and see what is going on below? If it is old filler having an effect here, what would I need to replace it with?

Alternatively, is there a type of paint that is can use that will be able to hide this? I have read that one can apply salt neutraliser to stop efflorescence then paint. Is that correct?

Many thanks for any help.

 

[Wayners]

Get a summer fan blowing on it. Not heat just air movement.
Should see a difference in a day. If it don't go or comes back you know it needs looking at.

 

[Sef1928172]

Thanks. Sorry. Should have said that I have had a crack at that already. That first pic is about as small as the spot has gotten after a good few weeks of warm weather and dehumidifiers and also put a hair dryer in it for a while.

So is hacking out plaster down to the brick the only option then?