Painting an exterior wall

[Spinney]

We have an external garden retaining wall (i.e. not part of the house). This is brick with a very thin rendering over parts of it. It is holding back a gravelled area that belongs to the flat above.

Part of it was painted last summer, but we ran out of paint and left the rest. There are no problems with this bit of wall.

We painted the rest last autumn, but now bits of the render/mortar are falling off. Some of this may have been loose to start with, but not all of it, I think. This is only happening on the bit we painted most recently. It would be too much of a coincidence if some change in the wall co-incided with the point at which we stopped painting in the summer, so it must be something to do with the conditions in which we painted it (we used exactly the same brand of exterior masonry paint both times).

Any idea why this happened? Is it anything to do with the paint possibly being applied when the wall was too wet? I don't think the weather was frosty at the time. If it's to do with being wet, the wall behind the good paint will also get wet from water soaking through from the ground above, so why hasn't that started to lose its mortar?

The good bit is on the right in the bottom pic, the disintegrating bit on the left.

 

[asterix]

Damp, innit? The coping bricks are very unlikely to keep the wall dry and the damp is getting behind the paint and lifting it off.

..and the paint is probably stopping the wall from drying out. In a freeze the trapped water may not do the wall a lot of good.

My barn coversion has stone walls and it is important to use a lime mortar, not Portland, in order to allow the moisture to escape. I have put a portland cap on the wall to stop rain getting in at the top but lower down it has plenty of area to 'breathe'.

 

[Spinney]

Damp, innit? The coping bricks are very unlikely to keep the wall dry and the damp is getting behind the paint and lifting it off.

Yes but...

The same amount of damp will be getting behind the paint on the 'good' bit of wall, which is not flaking off.

And it's not the paint that is coming off, but chunks of (admittedly slightly dubious) mortar.

 

[asterix]

Which way does the wall face? My 'problem' wall is SW facing - the way the rain comes.

 

[rodgy-dodge]

Yes but...

The same amount of damp will be getting behind the paint on the 'good' bit of wall, which is not flaking off.

And it's not the paint that is coming off, but chunks of (admittedly slightly dubious) mortar.

There may have been moisture in the brick to begin with, before the paint was applied. Then with a frost it will explode! all you can do is try and remove it and re-render it

 

[rich p]

Coincidence I think Spinney. I can't see that there is anything in the paint that will affect the mortar in that way.

 

[Spinney]

There may have been moisture in the brick to begin with, before the paint was applied. Then with a frost it will explode! all you can do is try and remove it and re-render it

So damp wall plus frost...

But the already painted bit must also be damp? Maybe just not as damp?

The wall actually faces the wall of the flat with about a 2 foot gap (the flat is a basement flat, so partly below ground level) - so it doesn't get rain blown onto it, all rain must soak down from above.

Our plan is to get rid of all the lose bits and repoint where necessary after a dry spell, and then let the mortar dry and then paint after a dry spell. Then we'll see... (but I'd still like to understand why there's a difference in the two bits of wall!)