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The fluffy salts is efflorescence, which is a natural occurrence when moisture is drying out beneath the surface. Assuming the source of the leak is definitely fixed you should just wait until the drying out process has completed before attempting to redecorate.

This could take several months.

Zinnser will block the stain coming through, for now, but it won’t cure the problem. You’ll need to remove the mould with a fungicidal solution and allow the wall to dry out in depth otherwise the moisture will just show up some place else.

There isn’t a cure, as such, since you would need to take yourself back to before you started.

Trapped moisture is forcing its way out hence the blistering and subsequent mould patches.

The best way forward is to scrape off any loose paint and allow plenty of time for any embedded moisture to work its way out – this may take a long time and I would certainly not attempt any repainting until late Summer. The mould should clear itself up once the source of moisture has gone but you can treat any large, persistent areas with a fungicidal wash

Patch paint with a regular water-based masonry paint (maybe thinned a little first coat to act as a primer), as you go. This may end up being a process you have to keep repeating or you may be fortunate and have seen the worst of it already.

In a domestic setting there is no use case for Pliolite paints and stabilising solutions should only be used where the surface is loose/powdery, and never as an overall primer.

The answer is to use a standard water-based masonry paint to protect the wall surface from the weather, this is what it is designed to do.

There are mineral-based finishes that create a ‘chemical bond’ which can outperform regular masonry paint but they are expensive, require professional application and wouldn’t really be worth it in this situation.

So called ‘specialist paints’ such as Wethertex make great claims about flexibility but it is a film of paint at the end of the day and is not able to withstand the kind of forces that cause cracks in walls – which is how water will penetrate a building, not, as they imply, through some state of osmosis that only other brands of paint are susceptible to.

Pliolite paints are formulated to dry quickly so you can use them when rainfall is imminent – which isn’t much of a solution to your problem I am afraid. They offer no advanced protection and will perform worse than standard paints over time for the simple reason that they offer a compromise which is only advantageous for commercial applications (ie, you can the job done in less than perfect weather conditions).

One a positive note, your neighbours extension is going to offer a significant degree of shelter to this wall which would be otherwise fully exposed. So may actually be of some benefit?

For small areas you can just dig out a small amount of filler and use coloured wax instead. You can mix together different shades to get a good match. Or use a touch-up pen.

For larger area, it’s not impossible but would take a lot of skill and patience, experimenting with different solid base colours for the filled areas so that when you apply a stain on top it matches (known as ‘woodgraining’).

Alternatively, try an opaque wood finish.

Blistering is usually caused by trapped moisture or surface contamination, it sounds like the latter in your case and is very unlikely to be anything to do with the paint itself.

It isn’t uncommon for such problems to only occur after the second coat because the containment is water-based and becomes soluble when the first coat is applied, and then migrates to the surface.

What I would so is scrape/rub down the blistered paint back to firm surface and wash down the area thoroughly before repainting. You may need to use a fine surface filler to get a level finish.

If the blistering is severe it may be worth using a shellac-based primer such as Zinsser B-I-N just to make sure the surface is fully sealed. Do this before applying filler and any more emulsion.

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.