I have done so much research but can't find an answer to this question. We moved into a house a few years ago with a loft conversion. The walls were pale cream colour and we didn't notice anything odd until we painted them red. Then the internal wall with the velux window (so, adjacent to the roof) started exhibiting this weird chalky substance. There was no damp smell and it washed off easily with light soap and water; however, it will return after several weeks.
I did some research and though it had something to do with the paint quality, so I bought a sealant, painted that on the wall, then added two coats of a blue coloured paint.
It seemed okay for a couple of months, and now it is more visible than ever.
I had added two pics. One is how it looks before trying to clean it off (fine white veins), the other show a square patch I cleaned and how it looked 1 month later.
Any ideas what this is, and how I can get rid of it?
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