Paint or render a low 300x100x500 limestone block garden retaining wall
Welcome to the Bunnings Workshop community @Splinter63. It's marvellous to have you join us and many thanks for your question on coating a retaining wall.
Before considering either paint or render, the first question I'd have is whether the rear of the limestone retaining wall has been waterproofed.
This is important because if the wall has not been waterproofed on the soil side, moisture can enter the limestone from behind. In an uncoated wall, that moisture typically migrates through the blockwork and evaporates from the exposed face, allowing the wall to dry out periodically.
When you apply a paint or other impermeable coating to the front face, you can effectively trap that moisture within the wall. As moisture builds up behind the coating, hydrostatic pressure can develop and push the coating away from the surface. This often results in bubbling, blistering and flaking paint, sometimes in a relatively short period of time.
Rendering is generally a better option as it remains porous and allows the wall to breathe. However, if you're looking to change the colour of the wall, I'd recommend incorporating an oxide into the render mix rather than applying a paint layer over the render afterwards. This maintains the breathability of the system.
Even with render, though, if the rear of the retaining wall is not waterproofed, you can still experience moisture migration and staining. Water moving through the wall often carries minerals, tannins and fine soil particles with it, which can discolour the rendered surface over time.
For that reason, if the retaining wall has not been waterproofed behind, I generally wouldn't recommend coating over it.
Out of interest, what is it about the limestone that you'd like to cover up? Limestone is typically quite an attractive finish in its own right. Is it discoloured, stained or weathered, or do you simply not like the look of it? If it's a condition issue, there may be cleaning or restoration options that would give you the result you're after without the risks associated with coating the wall.
Let me tag @emmawigg, @Loges_8, @Heffa76 and @JustTryinMyBest, to see if they have any suggestions.
Please let me know if you have any questions.
Mitchell
I am looking Porters Original Paints - Mineral Paint
My retaining wall has a lot of water that filters through after heavy rain so I need the product to be breathable
https://www.porterspaints.com/media/bmfpmq53/mineral-paint-specification-v13.pdf
Hello @Heffa76
Thank you for sharing the paint specifications of Porters Mineral Paint. It is possible to apply Porter’s Mineral Paint over a limestone wall with moisture issues, provided the moisture is in the form of vapour rather than running liquid water. Because it is highly breathable, it allows water vapour to escape, which prevents the paint from blistering or peeling.
Mineral paint will handle breathing through dampness, but it will not fix an active water leak or rising damp. You must fix the root cause of the water ingress before painting as advised by my teammate. If moisture is bringing alkaline salts to the surface (white powder), these salts act as a bond-breaker. They must be dry brushed off and washed with a mild 5% white vinegar solution prior to painting. Please make sure to rinse the surface very well prior to painting.
Do not paint directly onto bare limestone without an appropriate basecoat. Apply one coat of Porter's Paints Mineral Silicate Primer (ordered at the special orders desk) to bare masonry, this penetrates the stone and chemically reacts to create a strong bond. My best advice is to make sure that the surface is totally dry. Please note that should the moisture come through the limestone during the paints curing period the paint will fail to bond with the limestone surface.
Please remember to wear personal protection such as gloves, goggles and a mask.
If you need further assistance, please let us know.
Eric
Hi Mitchell, I like the look of limestone however the blocks are dirty, some green/black in colour. My first choice would be cleaning if you have a product/method to clean them. I also have a large retaining wall with the 1000x 500x 500 which are very dirty and have not got any better hosing them down with a pressure spray, karcher bought at Bunnings. Decent water pressure. Can you help with tips on cleaning. Limestone walls. They are not sealed behind. Cheers Splinter
Hi @Splinter63,
Whichever way you decide to go, whether that's rendering or painting, the retaining wall will need to be cleaned first, as the current dirt, algae and grime will prevent any coating from adhering properly.
Since you'd prefer to keep the natural limestone look, I'd focus on cleaning it. A pressure washer is the best place to start, but I'd recommend testing a small, inconspicuous area first to make sure the pressure isn't damaging the surface of the limestone. It should remove the vast majority of the built-up dirt, moss and algae. I'm surprised that one didn't have any effect, as typically walls in this condition come up remarkably well even with just a pressure washer. If you are seeing no effect at all, I'd be questioning whether the Karcher is actually functioning correctly.
Once you've removed as much as possible mechanically, I'd follow up with a product like the https://www.bunnings.com.au/30-seconds-2l-outdoor-cleaner-with-rapid-hose-end-technology_p0130860 . Apply it as directed, allow it to work, then pressure wash the wall again. I think you'll be pleasantly surprised with the results and I suspect the blocks will come up looking almost like new. If the chemical cleaner doesn't improve them, then you'll still then be prepared to look at coating the walls.
Please let me know how you go, and feel free to post some photos once you've given them a clean.
