I decided to paint a bedroom wall and pulled a small wardrobe out of the corner.
Found black mould growing in the bottom corner and up the wall and a couple of spots of peeling paint. There must be 50+ layers of paint on these old walls and it's at least 2mm thick and not the first wall to develop a small "peel".

Anyway, I got the scraper out and scraped away a bit at the peeling sections and realised that the mould is on the layers of paint but, the plastered wall underneath is bone dry and perfectly clean. Why is that? I thought damp came from outside and this isn't a room that gets loads of condensation.

If you look at the photo you can see where I've scraped and that the wall is clean underneath.

Can't paint now until I've finished scraping and yes, I am wearing a mask. Am I supposed to treat the freshly exposed plaster with anything before putting paint on it?

You can get specialist paints that inhibit mould growth. Could be caused by condensation if the walls get cold - try a moisture trap in the room too

Probably condensation. Buy a moulder buster spray or wash it down with watered down bleach. Let it dry. Use a paint for bathrooms or kitchens or add an anti mould additive to regular paint. Use a good quality paint not a crappy watered down Brico etc. Surprised you haven't got mushrooms growing with all the rain this year.

Thanks peeps.
I've only ever used good "breathable" anti-mould paint. God knows what the old owner used on the walls though. He was a DIY nightmare. He painted sliver-grey marble a dirty, shitty brown colour. Suppose he wanted it to look like wood. It just looked like crap.

Will check corner carefully for mushrooms coz you're right about all the sodding rain. Surprised I haven't got mould growing out of my ears.

We've got mould like we've never had it in 8 years. Sprung a leak in part of the building we don't use much, and water was a cm deep before we found it. Walls sucked up all the water, then the mould appeared, up to waist height. Nothing for it but to scrape off all the old paint, and yes, the plaster was fine underneath.

We use "Viamuffa" - not cheap, but used it in another part of the building, must be 5 years ago now, and no sign of mould at all.

hi,i think its just in areas where air can,t circulate we pulled chairs,units etc an inch or so from wall,that seemed to work.we used domestos spray bleach from uk,not diluted.anyone know if there is an equivelent over here,cheers brian.

You are absolutely right Brian, mold seems to grow in corners where there is no air circulation, and especially if it is a corner where the walls are external.

I have read that it is better to use anti mold products than bleach as they kill the mold spores as well.
Check any place that sells paint, they sell good quality anti mold products which are not cheap, but work.

Laser 23 - Boero

The wardrobe was in fact in a corner of two external walls. No mould on the back panel of the wardrobe anywhere. There is a "proper" paint shop up the road from me where I usually get paint and sometimes art supplies. They sell other stuff too so I'll ask there for a good anti-muffa .... and then I think I'll put the wardrobe somewhere else.

Air circulation is the key to the solution, once you have that you can look at the right type of paint etc etc. If humidity is particularly high in the room then you will need to either air it, (open the window(s)) or put a de-humidifier in (a portable one will do).

good post air is good and costs nothing

We had a very similar problem and I took all of the paint off back to the plaster, sandpapered it, leaving it rough then applied a thin layer of plaster 'stucco dry' and then a coating of paint called 'xilolife'.

All is still looking good after 6 months.....

PSG

As per Linseed oil, this is all the advice you need.

My first experience of the problem was as a child, when I had a small closed-back bookcase against the wall in an exterior corner of my bedroom, in the UK. I had to relocate the bookcase and repositioned the electric radiator there. The problem never recurred.

I have since seen evidence of the problem in several houses, but have never caused the problem again because I make a point of never putting large of closed-back furniture against an exterior wall, and especially not in a corner. Some combination of air circulation, warmth and light always prevents the conditions for mould to grow.