Crack Filler Query

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Old_Geezer | 10:14 Fri 14th Aug 2020 | DIY
12 Answers
Suggestions for a flexible wall (plaster really) filler needed.


When I bought and moved into my present property I soon noticed what I thought was a rather dodgy bit of wallpapering in one area. As time went on and it became more defined it was clear there was something else causing the marks. Eventually I checked and there was a crack running down the wall. Being somewhat alarmed at this, I had a professional (surveyor I think) who said that it was not a foundation issue, as I had feared (cracks don't run into the foundations, no cracks from the doorways or windows, no broken tiles, etc.) but was down to dissimilar materials having different rates of expansion with temperature. I have no knowledge on the subject, so take that as being correct. The professional said I should get a builder to fix the cracks visible on the outside and leave the inside until I next decorate. No great hurry.

(On a tangent issue, I did try to get someone to do the outside but few wanted to look, the two that did, one only wanted to do the whole outside in something and wasn't heard of again when I said I only wanted the cracks fixed/sealed/whatever for now; and the other seemed pleasant enough but never put in any formal quote either. Darned nuisance trying to get anyone to do anything, let alone a capable person.)

Well, today I peeled off the surrounding wallpaper and see that a previous owner (I'd bet it was the one I bought from) had tried, in places, to fill this crack before. In a couple of places their filler came away. I need to get it fixed permanently; at least on the inside. (Suggestions regarding getting someone to do the outside can wait a while, but would also be welcome.) I'm assuming one doesn't need to dig out the blocks behind the plaster.

I don't need a filler that's inflexible and likely to let the cracks reappear over a year. Needs to be permanently ok under the paper/paint when I redecorate. So I'm asking anyone with building experience what they would suggest I use to fill it. Or general advice on the necessary tasks involved.



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Hiya OG.
Could you tell me the approx. age of the house, and the wall construction.
Cavity/brick/block/solid stone?
Question Author
Hi. Unsure of the house age without digging out the paperwork but it would have been around the time of WWII. The part in question is an extension though, no real idea how old; as a pure guess I'd suspect, say, 2005 ? and seems to be breeze block with a cavity.
A common problem OG. Differential expansion often shows up near the junction of the extension and the house. Also windows and openings where lintels have been used (junction of differing materials.) Although in your case, there is no problem near windows and doors.

If not that, then most likely shrinkage cracking, although I would have expected that to have shown up before now.
Anyway, assuming that there is no ongoing movement, then filling should be all that is needed.
For the interior ... use a simple flexible decorator's caulk.
Most people confuse "filler" with "caulk". Fillers tend to be rigid, so they can crack again.
Caulks are more forgiving. (Plenty of them available... applied from a 300ml tube using a cheap gun. I'm sure you know the type of gun I mean.
They do say they are able to be sanded down. I'm not so sure about that. Since you're papering, flattening it down with a damp sponge will do it.
Even with a thin crack, I find it's best to be quite aggressive and rake out to around 2 to 3 mm. Poke the gun in and squirt plenty into the crack. That's what helps stop it from coming out again.

For the exterior ... is it rendered with sand & cement?
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Not so much between the old and new, but more at various points along the new.

I have dug out the inside crack to various depths, 3 to 4 mm where the plaster came away with ease, about 1 mm where it seemed reluctant. But I can go make it equally deep.

Caulk eh. I'll see what the local DIY sheds have, unless you know prefered brands ?

There is wallpaper up at present, I'm still deciding how to put it back together after filling. I had hoped just to paint over, but I guess that'll be too visible. Ho hum... Perhaps I can just repaper that external wall. Then paint.

The outside is rendered with something. Looks like paint covered pebbledash to me.

Thanks for the advice.
This looks really *** and is quite sloppy to use. But I will vouch for it. Easy to sand, Never had any comebacks. Used it on my own home. Never cracked. Better than any pollyfilla i've used. Ready mixed. Great for small repairs around socket boxes etc. I use Bonding and Pink finish for large chases, But I think this will do you just fine. I've been very happy with it !
A little pricey for a tub of this size. But it is a cracking (no pun intended) piece of kit

Ps. It's more of a joint filler like dry lining filler than a plaster. I certainly wouldn't recommend it for plastering !
Sounds like you're well on the way OG.
Yes, caulk is meant to be painted over. As I said above though. it can be tricky to "lose" it enough to paint over. Quite often, you can fill, smooth maybe with the sponge, prime it with a thin "mist" coat, then give it a good coat, let it dry and see what it looks like.
If still visible, then try rubbing down.
Painting over repairs is often difficult. I would be interested to see if others on here might offer their suggestions. Some of them do a lot more decorating than I do.

As for the exterior... same thing, but use "exterior" caulk. As with interior, there are loads of good brands. You can't go far wrong whichever you buy. They all do the same thing.

When you come to re-paint the external crack, it often helps to add some fine sand to the first coat. That really does help obliterate the repair.
Question Author
Cheers. Shall do, when I get around to it ;-)
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Re the mixed plaster, I think I'd be a little concerned how flexible it was. However I may have another job elsewhere that it might be useful for. Cheers.
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Hopefully this thread isn't so old that it gets updated unnoticed.

Yesterday, used the caulk to fill the crack, which I'd previously opened up. Learnt about caulk anyway. Issues though.

As I squeezed the caulk into the crack and used the filler/spreader/scraper tool to smooth it out, I found that the caulk in the crack seemed to shrink back every time, even after multiple applications. I eventually decided to overfill in the expectation of sanding off excess today.

First off it still shrunk back in places and I had to go over it again, and secondly; it's clear that how caulk manages to prevent the crack becoming obvious again, it is of a rubbery nature. But this means it is unlikely to react well to being rubbed down with abrasive paper; or am I mistaken ? Maybe that is the way to go.

Or alternatively, can one get wide sharp blades that will let me easily slice the excess off at the level of the top of the plaster ?

I'm unsure of the best next step.

You have my sympathies, OG. I'm absolutely hopeless at decorating. I can build a whole house, but still get someone in to decorate ;o)

Anyway, in a perfect world, I would strip off all the paper and re-skim using reinforcing woven tape over the various cracks. (Scrim cloth/decorator's cloth.)
But, for you... level the final coat of caulk with a scraper, and let it dry and go concave.
Then I would finish with a fine surfacing filler (readymix tub.)
The type of filler that dries and CAN be sanded.

If there is still a problem, try this before papering...;ds_rl=1241687&ds_rl=1245250&ds_rl=1245250&gclid=Cj0KCQjwufn8BRCwARIsAKzP696ckVO_lZhYJ8w3JTeFtpoqo1mX4jChMJd4pXCyU6HJAgJgL8HrRRMaAjIWEALw_wcB&gclsrc=aw.ds

Perhaps some of the proper decorators on AB can comment :o)
Question Author
Thanks. Unsure when I'll be there next what with this lockdown nonsense but I'll give that a try when I am.

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