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Painting over it with a thinned-down coat of vinyl based emulsion will bind the surface together but, as you suggest, the existing coating is not properly adhered to the surface and will come away eventually.

It really depends how bad the problem is and whether you can live it?

If you are able to wash it off then that is the ideal answer. One other solution is cross-lining the walls with lining paper but it would need to be done professionally and isn’t a permanent fix.

Water stains tend to be light brown in colour so it is indeed likely the stain block that is showing through. You can get this when using different kinds of paint and usually a couple of extra coats will do the trick.

Do two coats just over the affected area to ensure it is covered and then do one final coat over the whole ceiling to finish.

I am not familiar with Wickes Stain Block but my first thoughts were that it is likely a water based product and, as a consequence, will have a limited effect on water based stains. It does say on the site that it is solvent based though.

The link to the product data sheet, however, is for another product entirely, so not much help there.

Another coat of the stain block you have might do the trick but, if not, I suggest trying a spirit based stain blocker such as Polycell aerosol stain block or, alternatively, a traditional oil-based white undercoat.

As with any kind of textured surface there is rarely a simple answer. It depends, to a great extent, on how widespread the problem is. Small, localised issues are easy enough to resolve with a scraper or, sometimes, a heat gun. Larger areas can sometimes be tackled with a high pressure washer and/or physical scraping.

Blistering is usally indicative of trapped moisture so you will need to also ensure the wall gets time to thoroughly dry out before attempting to repaint.

When you say the walls were painted a few months after being plastered (rendered) I think the problem may be that that the render had not dried out fully in depth and this can sometimes cause problems such as you have described.

I’m not familiar with the term ‘mapping’ but you often see what you describe happen on new build housing where the painting has been completed more or less as soon as the walls have been newly rendered.

Since you say the coating seems OK otherwise I wouldn’t worry about it. People sometimes suggest the need of a primer because it obviously increases the price of the job.

I wouldn’t recommend Farrow & Ball for exterior use though since their formulations are weighted towards giving a rich aesthetic effect rather than long term performance.

Sandtex or Dulux Weathershield will perform better, even if you have to compromise slightly on your preferred colour since the matching can never be 100% accurate due to the different formulations.

The release of harmful fumes occurs mostly during the painting and drying process. Therefore, so long as the room is well ventilated and you leave sufficient time between painting the bed and and its eventual use, the long term effects are minimal.

If you want to be absolutely safe though, you can indeed paint again with a low/zero VOC paint and this will have the effect of sealing-in the previous finish.

Not much I can add other than what you’ve already been advised, which sounds fair. The paint is only going to be as good as the surface you’re applying it to. So any loose material and mould needs to be removed before you do anything.

You don’t give any information about this wall or its construction/design, so impossible to give any further advice on this.

As for applying a stabiliser, there is ample information already here that you haven’t read so I suggest you start there.

There are a few specialist manufacturers of exterior paint that claim to me mould and dirt resistant but I am not not going to recommend any of them since a lot depends on the type of surface, its current condition and location as to whether these paints have any chance of providing better protection than a regular masonry paint.

There is also no factual information that I am aware of that proves any of these paints actually work, other than their own publicity.

The paints you mention will perform well as a surface finish but your biggest problem is obtaining a successful bond with the granite surface to begin with.

First you’ll need to remove any shine from the surface with fine wire wool and thoroughly degrease the surface before painting.

A surface preparation treatment such as Owatrol ESP will be your ideal first step. This will help any subsequent coats to adhere to the surface without the risk of peeling or scratching off.

Also bear in mind that most products you use as a top coat will have a curing time that goes far beyond surface drying times (sometimes a few weeks) so you’ll need to avoid the risk of damage during this time until the paint has fully hardened.

The exception to this rule is with with some industrial quality 2-part products which will cure more or less straight away.

Hard to say without seeing the problem but such problems are usually caused by either condensation or trapped moisture.

It could be that your old windows were a bit drafty but were also providing adequate ventilation to prevent the build-up of condensation in this area.

Or, they have used a hard, non-breathable damp-proofing plaster and/or paint finish which is holding in moisture behind the surface. This could cause a cold-spot which is allowing moisture in the air to condense on the surface – hence the black mould. This would also explain why paint is flaking-off because the trapped moisture is forcing its way out.

As I say, pure guesswork on my part, but you will need to get to the root of the problem before attempting to redecorate.