Cracks in plaster

Participant · 3 May 2019 at 06:56

Hi all

I'm about to redecorate our hall stairs and landing. It'll be a big job with lots of prep work to get a decent finish, one of which is to sort quite a few hairline cracks in the 1960s plaster that's since been skimmed few times.

Having watched a few YouTube videos, my plan is to Dremel / pad saw the cracks to widen them, fill them with expanding foam, cut away the excess foam with a scraper, then use two coats of Gyproc Easifill 45 with scrim tape to hopefully make a decent finish with no returning cracks, at least for a while.

Did this approach sound ok? Had anyone had success with other methods or products? Unfortunately taking it all back to brick isn't something we want to entertain.

Thanks.

Hedge · 3 May 2019 at 07:15

Having to use expanding foam in something that was a hairline crack seems extreme.

Won't a decent quality filler be enough ? Just sand the crack and use a scraper to make the crack slightly wider.

I might be wrong but I've used polyfilla etc in past and cracks haven't came back.

200sols · 3 May 2019 at 07:29

I’m struggling to picture you filling a hairline crack with expanding foam..

NickK · 3 May 2019 at 09:20

I’ve become a bit of a master at this based on experience unfortunately.

I investigate first by tracing the crack back as far as it goes. I’ve uncovered cracked concrete between blocks and cracked blocks. Often I’ll rip out back to the blocks, fix with new concrete then rebuild with base plaster and then finally finishing plaster.
I’ve found a hairline was just the tip of a problem where the cracked blocks were the root cause. If it’s just plastered/painted then it simply cracks again in a year, rebuilding solves that (at least for a couple of years if not permanently).

This is with paint rather than paper.

Rip back, concrete, base plaster, finishing plaster with PVA on each surface, sand back to flat using 80, 120, 320. Next two coats of pollyfilla plaster with a roller, then sand fine to smooth and the paint layers on top.

Participant · 3 May 2019 at 09:57

Thanks for the tips.

200sols said:
I’m struggling to picture you filling a hairline crack with expanding foam..

It starts out as hairline but isn't so hairline once the loose plaster has been removed. E.g.:

And one I haven't attacked yet...

200sols · 3 May 2019 at 10:03

Interesting, they go quite deep.

Mercenary Keyboard Warrior · 3 May 2019 at 10:12

If they are quite deep it could be signs of a diff issue. I would personally do it properly, layers of proper filler then a fine surface filler on the top.

Looks like a lot has been gouged out there, no need to go to that extreme just to fill it.

The only crack I have ever seen that would never be filled was one in my grandads house. It was a structural issue, basically it needed a bit of underpining but he wasnt that worried (had lived there 40 years with no drama) and as an ex council house it was an insurance job or pay himself. It was deemed relatively minor and as such never fixed. But it meant it was impossible to have that bit of wall perfect, it would move and as such it would crack plaster, rip paper etc.

I would take some aggressive sandpaper to that crack, on a flat block, and see how it reacts to the sanding. Plaster is very soft so its easy to remove far too much with something like a knife. You can also get flexible filler specifically, expanding foam is a nighmare, its very hard to get just the right amount, it tends to have air in it, and is a bit of a pain to smooth. If you have a lot of deep cracks to fill then maybe patching plaster as its a lot cheaper than filler.

Participant · 3 May 2019 at 10:29

The reason I took it back so far is that I watched a YouTube video saying that expanding foam can get behind any blown plaster not attached to brick and re-adhere it to the brick.

I've tried sanding / filling / sanding some of these cracks before and the cracks reappear.

I'm not concerned that it's structural tbh as there are no new cracks appearing nor are the cracks getting worse in 5 years in this house, however it's possible I guess.

phrases · 3 May 2019 at 10:34

I'd just use multi finish it will bond well enough just leave it 3mm proud of the surface so you can go over it again to blend in with original plaster.

You could knock up some bonding and remove that chunk of plaster that has blown isn't that hard but a little awkward. Again leave it just proud 3mm then over it with multi finish i would practise on a piece of plasterboard though first if you are new to plastering as its quite easy to go overboard when blending plaster just keep spraying water as you go towards making it flat.

Mercenary Keyboard Warrior · 3 May 2019 at 11:04

Participant said:
The reason I took it back so far is that I watched a YouTube video saying that expanding foam can get behind any blown plaster not attached to brick and re-adhere it to the brick.

I've tried sanding / filling / sanding some of these cracks before and the cracks reappear.

I'm not concerned that it's structural tbh as there are no new cracks appearing nor are the cracks getting worse in 5 years in this house, however it's possible I guess.

Well it might thats true. Its horrible stuff though, as i said quite tricky to get the right amount. If you have blown plaster and get too much behind it will make it worse, its quite impressive the energy it manages to generate.
I saw it pop a skirting off at a mates, he squirted a load into a hole and then nailed the skirting in place, plus grip fill. 30 secs later the skirting pushed itself off the wall

NickK · 3 May 2019 at 11:08

I’d be inclined to take out any blown plaster. My preference even if there may be an easier way. Always concerned with expanding foam as they say it’s carcinogenic. The kitchen builder used it and it works but it still feels like not a proper job done to me.

Mercenary Keyboard Warrior · 3 May 2019 at 11:27

NickK said:
I’d be inclined to take out any blown plaster. My preference even if there may be an easier way. Always concerned with expanding foam as they say it’s carcinogenic. The kitchen builder used it and it works but it still feels like not a proper job done to me.

Its a quick way to botch a job. Gaps round blockwork, holes to fill etc.

Personally I am always the same, cut back to good and then fix. If its really blown over a large area then rebuilding from scratch will give a solid foundation. I am always wary of things like blown plaster, just takes the wrong thing to happen and you have another issue. I wouldnt be spending significant time fixing up without doing it properly personally.