Fix Small Patches of Crumbling Redering


Postby DarkMavis » Sat Nov 17, 2018 4:31 pm

I'm painting a relative's house and on the West facing wall there were a few patches of flaking paint. The rendering underneath was a little soft, but not terrible, so I just used a pressure washer to get back to something solid. In some places, just the paint came away, taking the tyrolean with it, in others, maybe a quarter inch of render came with it, but we're talking about maybe two or three patches of a square foot or two.
All these patches are quite small - most are less than a square foot, and in most cases, the rendering is fine, only the tyrolean and paint came away.
Since I'm dong this for a relative, I want to keep the cost as low as possible, so I can't justify hiring a cement mixer and mixing render, since I'd only use a fraction of the materials.
How cheaply can I do this?
I'm thinking of just buying a bag of ready mixed render, a bag of tyrolean and hiring a flicker from Jewsons, but there is already half a bag of Everbuild all purpose filler here. I'm wondering if that would suffice, and whether tyrolean would adher to it? It's a wall that nobody can even see, so they may not even care if the tyrolean is skipped entirely. I'll have to ask about that
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Postby DocMartin » Tue Nov 20, 2018 12:14 am

I've just looked up the Everbuild filler and it says it is suitable for exterior render patching. In any case, given that you only need to fill a few square feet of render that's fallen off to the depth of 1/4", I wonder why you mention hiring a cement mixer? You can mix up a bucket of mortar easily enough without using a cement mixer. You'd use a cement mixer for laying a foundation or similar!

If you do fill the holes in the render, make sure the surface you are rendering onto it sound and not dusty, else it'll fall off. If the base material is crumbly or dusty you can stabilise it with something like diluted exterior PVA or indeed there are other stabilisers (except they tend to expensive as you have to buy a big tub).

If you go the cement route, Readymix might be OK. I'd say exterior render might be a 1:4 cement:sand ratio. Don't use 1:3, too strong. Not sure what ratio Readymix renderis. It's not critical. I see B&Q does 5kg tubs of this. Guess what - it says on the front you need PVA! It costs just over £8 for a tub of it, and that's buy you a bag of cement and a bag of sand anyway. As for the PVA - I guess that's to plasticise it (make it handle better ) and stick better to the surface being rendered.

Use a straight edge (decent piece of wood would do) to make sure the new render is flat. Scrape off the excess with a straight edge if the render is bellied out, add more if it's sunken. Getting the right amount of water in the render mix is critical, too dry and it falls off and crumbles, too wet and it slumps. You may need to press it hard with a mortar trowel at first to make sure it sticks to the substrate. I have used a thin, slightly wetter mix for this, in difficult cases, then go over it with a slightly dryer mix which holds its shape, but a decent render wouldn't have any trouble doing it in one coat.

As for the Tyrolean - easy to apply if you have the consistency right I understand (try it on a bit of plywood first) but how will you match the colour?
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Postby DarkMavis » Mon Nov 26, 2018 3:27 pm

Matching the colour isn't a problem, since it's being painted over anyways. I think a solution has presented itself, though. There's a large unrendered wall around the garden, so I mentioned that if I were to render that and apply tyrolean to match the house, then the whole job could be done more efficiently. They've been planning to get it rendered eventually, so doing it at the same time as these small patches is the right thing to do.
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