How to stop a patch of bubbling paint

[swinster]

Hey all,

I have recently had basement renovation with most wall having dot/dab plasterboard, apart from a couple of cheeks of an alcove that were plastered direct on the brickwork. On one of these cheeks the paint bubbles an falls off in a smallish area (approximately 30cm sq). I have painted the area a few time now but after a couple of months tha paint starts bubbling, and eventually I will need to scrape off all the paint in the area.

Is there anything that might be causing this and anything I can do to stop it?

[Tabby]

damp. and no, if damp's coming through a basement wall there's nothing that will stop it. Suggest redoing that bit in dot & dab, then the damp can evaporate away.

NT

[swinster]

I don't think its damp. The "basement" is actually at ground level at the rear of the house, the alcove is part of an internal wall (terraced house), and the problem is actually on the cheek of the alcove. The alcove is created from the chimney breast, so unless damp is coming in from the very top (4 floors away), I just can't see it being damp. The patch that is bubbling is around 5-6 ft from the floor (about a foot below the ceiling), and there is no problem either above of below this patch or on the other cheek. The plaster underneath doesn't appear to be damp - there is no tell tale dark patch - however, is there anyway to tell for sure?

Further info. I painted the plaster with a 50/50 mist coat emulsion, then when dry with an acrylic eggshell (water based). However, there must be an issue with this particular area as this is the second time it has happened.

[swinster]

PS. this area is actually plaster on brick - the rest of the basement in dot/dab. I had to remove the dot/dab on both cheeks to give myself just enough room to fit a range cooker between the cheeks in the alcove.

[Tabby]

oh I see, I though this was underground external wall. For want of a clearer direction I'd strip any that will come off and redo it properly, 1:5 pva first, then emulsion and only emulsion. Piss coats result in much of the glue in the paint migrating into the plaster, much weakening their hold. And I'm not 100% certain how reliable acrylic on emulsion is. I dont know if looking inside a bubble might tell you which layer is detaching.

NT

[swinster]

Cheers. I thought I had PVAed first originally, but I might be mixing it up with another wall!!

I did check with the supplier about the compatibility prior to application. I was told that as both paints (emulsion and acrylic) were water based, there would be no issue. Indeed, in the rest of the place the paint as gone on very nicely.

I do need to scrap off some more paint in the next couple of days (after Christmas has died down) so I will check the bubbles, but I'm pretty sure the paint is peeling from the very base layer to the plaster - when it all comes off the plaster is showing and its not as though I'm trying that hard to scrape off the paint to the plaster.

[Tabby]

Sounds like the piss coat failed.

NT

[Tabby]

[The Natural Philosopher]

damp.

let me guess, its an old chimney breast? My old house was permanently like that.

Only remedy is to somehow either stop the damp getting in - a ticklish project - or strip the plaster and render with string mortar - 50-50 sand/cement - and then replaster. That will drive the damp to hopefully the inside of the chimney where the flue can evaporate it.

Or you MIGHT be able to get away with a DP injection into the brickwork itself.

Followed by replastering etc.

[The Natural Philosopher]

well look again

Its either rain coming down the chimney, or if its low..below a ft above ground level - its damp rising through the stack. from the ground. Most chimney stacks were never damp proofed. The fire in them would keep the things dry enough.

The patch that is bubbling is around 5-6 ft from the floor

meter maybe, but this looks more promising. At that height its likely to be rain ingress into the stack - there's probably a brick ridge inside that collects water or something.

strip the plaster round that area - for at least 6" every way and apply a strong mortar direct to the brick, then replaster it.

[swinster]

Cheers. I think I will hire out a damp meeter for a day just to double check.

I should also be able to access the top of the brickwork of cheek of the chimney breast (just above the ceiling of the area that's bubbling) as we still have a big hole in the floor above where the old hearth was knocked out, and a hole in the ceiling in the middle of the alcove where the a new kitchen hood extracts to.

All a bit difficult to explain without photos.

[Tabby]

I think that would be pointless. The measure of whether you have a damp problem is not the measure of water content plus other causes of conductivity, but simply looking at whether you have the signs of a damp problem. Damp and damp problem are 2 different things, old houses routinely have some damp plus greater conductivity. Also you've just put plaster and paint up, so the cause and possible permanentness of any damp would be unknown, making the readings of little use.

You already know the piss coat failed, and that much of the glue in it was washed out on application.

TNP's advice is contrary to that given by the experts. Check out what SPAB says about cement on old brickwork.

NT

[swinster]

On 28 Dec, 14:49, Tabby wrote: . Check out what

Are you referring to "

" ?

I realise that these comments are outlined in relation to old/historic buildings, and whilst our house is quite old, it still only dates to the 20th Century (1914 ish). Our house (along with most terraced houses round here) are rendered externally with, I assume, cement renders and pebble dash. Interesting comments nevertheless, but I'm not sure how relevant they are to this situation.

[The Natural Philosopher]

No it is not

Its just against certain stupid people who think that the cases of very old brickwork built out of lime mortar and with inadequate damp proof courses are universally applicable.

Waterproof mortar tanking is a well known technique used in many victorian/edwardian properities to good effects.

A si brick injection below normal DPC.

Check out what

Check out what SPAB has to say on global warming and teh keeping of bees.

It has as much relevance.

[The Natural Philosopher]

They have no relevance. It's Tabby's bonnet bee. To him there are only modern houses and those built before 1700. Out of damp crumbly brick and lime mortar.

The fact that Portland Cement and fairly hard brick have been about since the turn of the century, passes him by completely.

[Tabby]

Theyre relevant to buildings of soft brick and shallow foundations.

Comments but no substance from tnp I see.

NT

[The Natural Philosopher]

No, that's you dear.

[Tabby]

If you have any expert reference that explains the issues SPAB et al raise regarding cement on soft brick with shallow foundations, feel free to link to it.

NT

[The Natural Philosopher]

Why do you persist in waffling on about this, when it has nothing to do with the original question.

What about the above do p yopu utterly fail to understand?

[Tabby]

didnt really think you had anyting