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Back in May last year I painted my recently rendered walls with Dulux Weathershield smooth masonry paint. I thought it looked quite good at the time - [URL= http://i35.photobucket.com/albums/d187/Inferno182/IMG_20160520_174242_zps4q7xbkgf.jp g" target="_blank">
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Unfortunately whatever it is B&Q used on the decking above the curved wall to give it its colour leaked out of it pretty rapidly when it rained. After a few months the curved wall had a lot of brown staining and looked horrible.
So I gave it another few months until I thought all of the deck colouring had leaked out of it and repainted the wall (using the same paint left over from last time). It looked good again for a day or so, but over time the paint bubbled and again looked awful.
So what's going on? A reaction to whatever leaked out of the decking? Could the rendered wall be damp or something? I want to paint again, but I would like it to be the last time for a while!
Damp you ask. On an bellow ground level wall with earth behind it?
What's the wall construction?
What's behind the wall?
What type of render?
3 question all with varying answer/problem ratios
The wall is made of aerated blocks, unfortunately I don't know the type of render used or exactly what's behind the wall.
I believe there was a pond where the lower decking is now and I think there may have been some sort of water feature feeding into it from where the upper deck now is.
The base for the upper deck had some sort of material covering it, but I'm guessing it's just mud behind the wall.
The blocks certainly won't help, they suck moisture tremendously. So as above it's probably a combination of penetrating damp from the rear and ground salts.
No paint is really going to resist that kind of pressure from the rear especially on thermalites or similar. It could really do with protecting on the rear side with bituthene or similar but you can't guarantee you won't get upward draw of moisture from the bottom course where it meets the foundation.
Did you clean the wall? Not my area but you may need to prime/seal the existing material if it has been allowed to weather.
Best solution is to remove and start again. Not sure how easy that is to do - I tend to work on steel.
Is it happening on any other wall or just the curved wall with decking above? If so what prep did you do before re-painting with the dulux masonry after the stain bleed. If its just that one wall where the problem is then I'd suggest it's contamination from the decking coating bleeding onto the render below
Ps, a possible cure would be to remove as much loose paint as possible, clean down with a mild detergent and then coat with pliolite masonry paint. This would stop the oil/water based reaction. If that's the issue that's causing the problem.