I purchased my top floor flat in a Victorian building and painted over the (refurbished) magnolia coloured interior walls. 2/3 years down the line the interior paint on the exterior walls has blown and erupted. Have treated it for damp/effervescent salts but stilll it keeps erupting! Any ideas please or will I need to have the plaster removed and redone on these walls?
Thank you

The outside walls must be allowing for moisture to penetrate through,causing the new paint to blister.

Try looking on the outside for any way for moisture to get through.Windows not sealed properly,bricks with broken mortar lines,flashing not installed properly or not sealed,old paint that has holes or caulk lines not closed,etc.

On an old historic building here in town(the Lincoln Highway Association's, National Headquarters.),the building is of stone,the interior walls are plaster over lathe,in certain areas of the exterior walls,the insides had blistered,the water that penetrates the stone façade allows too much moisture through, and those walls are up to 12" thick.Imagine what a 6" wall might allow if even the smallest opening was not sealed.

Another thing to consider,do you have any pipes in the walls,water lines,exhaust pipes(dryer venting),stacks for the plumbing?

If the pipe boots have failed,water will find it's way to any 'level' surface and stand there.(also chimney flashings)

Good luck.