Salty fluffy stuff under plaster

[Roberto2019]

Dear all, currently decorating a bedroom and above the main window the plaster was bulging and generally not looking good.

I have knocked a lot of it off, and underneath there is a lot of salty residue that looks like it has literally pushed the plaster off.

The house is 1950's with a cement render over the brickwork and then a plaster skim on top of that.

There is also a large concrete lintel above the window so perhaps the saltyness comes from that.

Anyway, I assume it points towards damp, but as far as I can tell this must have happened a long time ago as it seems completely dry now, even after rain.

So, question is, is there anything I can treat the wall with to effectively seal in all the salty stuff to allow me to plaster back over the top of it?

Or, do you think just clean it all off and plaster over without doing anything else?

Many thanks

 

[bobasd]

photos would help?
do you have external render in good condition?
do you have cavity insulation?
does the room have ventilation and heating?
google pics of "efflorescence"