Emulsion-painting mortar rendering - how to stop paint peeling off after a year

[NY]

Our house is built of bricks which have a mortar rendering that is painted white.

That part of the house was built in the late 1990s so it will have a damp proof course.

The paint has started to peel off the mortar in patches. I repainted it last year with exterior emulsion paint, using three coats to give a whiteness that matched the surrounding area, without the sandy-grey colour of the mortar showing through.

The mortar is intact: there is no sign of it bubbling or coming loose from the underlying bricks. And it doesn't feel damp. I painted it in the summer when the weather was warm (but before last year's heatwave!) and when there had not been rain for a week or so.

I'm going to repaint it again. Is there something else that I should apply to the bare mortar before the first coat of emulsion paint, so the paint does not flake off again?

Ironically, the paint on the older part of the house (1800s, probably mortar over stone, no damp proof course) has not suffered in the same way.

[The Natural Philosopher]

Use a better paint

[NY]

Such as? I'm using Sandtex Ultra Smooth Masonry Paint. The instructions say "little preparation is required" and don't mention the need for any pre-treatment or priming of fresh masonry surfaces. My surface is not "fresh" in the sense that has has previously been painted, but the paint has peeled off without leaving any residue or pigment on the bare mortar. The mortar feels dry. When I tap it, I get the same sound over various parts of the wall, whereas if there was localised peeling of the mortar off the brickwork I'd expect a hollow sound in those places. What does it take to make masonry paint stick to masonry (mortar rendering) without peeling off in large flakes after a year in certain places?

I've noticed on other parts of the wall (*), the paint looks as if it has partly washed off, in the sense that the pigment is less intense white and has small random patches that are bare - but there is no evidence on the ground of large flakes of paint, whereas there is on the part that I'm originally talking about. But maybe the root cause is the same: something is preventing the paint making a good bond with the mortar.

(*) An older part of the house: probably still brickwork under the mortar rendering.

[The Natural Philosopher]

Think I used dulux weathershield. After 25 years there is a bit of flaking where water has got under. Its rated for a 20 year life.

[NY]

And Sandtex is rated 15 years - so still vastly in excess of what I'm getting.

(the bricks give an idea of scale) shows a huge area between and below the windows where paint flaked off last year and I wire-brushed back to solid paint before repainting the now-bare mortar with a couple of coats. Some of the area that I did is starting to bubble and you can see where patches have flaked off. I don't have a damp-meter to check how damp the mortar is where is is bare, but it feels dry: if I rub my hand over it, my fingers don't come away damp, and blotting paper pressed onto the mortar doesn't show dampness. And the mortar isn't peeling from the bricks, as if might do if rainwater was getting in through cracks in the mortar and collecting between bricks and mortar. I painted with a brush, working the paint well into the boundary between good paint and bare mortar.

Is there anything else I should be doing to prepare the wall before repainting (yet again!)?

[The Natural Philosopher]

Hmm. I have a few areas like that. I'd pressure wash all the loose stuff off before repainting. I did have a fair bit like that on a north wall where I left a tile on the gravel and the water splash was simply soaking the wall and then the frost did the rest.

ISTR I painted some sort of weather sealer on the render before I repainted it

[Brian Gaff]

When I had my house done in Wallcote many years ago, they used some paint that was kind of darker than the render. Nothing has come off since. I also have a freshly rendered wall recently renovated. the guy doing that painted something on it first before doing the main coat, but actually, in another place I am told the render there actually looks attractive in its natural form, so why bother to paint it? This seems to suggest something about the black art of making render that stays looking good and does not blow where it bonds to the wall. In your case of doing the mortar around bricks, that seems a fools game. If you have a foolproof fix for that, then you will make a fortune. I had my bare brick rendered and just was careful about the area of the damp course. I do however feel that in recent years the quality of ordinary paint, and even those specifically meant for walls is poor. I guess if it was too good nobody would buy their paint again!

Brian

[alan_m]

Wire brushed with a nice clean brush or a brush re-purposed from cleaning up some greasy car parts?

I would be tempted to wire brush back again, use an random orbital sander to feather the edges, wash down with detergent followed by a hose down with clean water.

[NY]

A clean wire "scrubbing brush" (flat slab handle with wires in clumps at 90 degrees to the slab).

I'll give it another go. And I like the idea of using a sander to feather the edges of the good paint. And cleaning with detergent avoids any grease problem. I wonder how long I need to leave the mortar to dry after getting it wet to wash it. Given that emulsion paint is water-based, does it actually matter if the mortar is still damp, as long as it's not dripping wet?

[Animal]

Cleaning is often not adequate, leading to premature failure. Emulsion is not normally exterior suitable. There are 2 good types of masonry paint, the most expensive & the cheapest. The rest I've not been impressed with. The cheap is lime.

[David]

Perhaps the mortar rendering is effectively waterproof and the paint is unable to bond? A while back, but IIRC advice for some paints was to thin 50/50 with water for the first coat to allow it to soak in a bit and form a bond. PVA could also be used to form a bond between the substrate and the covering.

Cheers

DAve R

[Harry Bloomfield Esq]

I used the rough finish version a decade ago, and it's been absolutely fine. The tin of Sandtex was at least 25 years old, stored indoors.

[John Walliker]

I have had bad experiences of trying to paint over surfaces that had been coated with PVA. The paint flaked off.

John

[Animal]

PVA is water soluble. I've seen it work outdoors but wouldn't count on it.