Cracks coming through after filling

[EdTurtle]

Hi, I'm a bit under-experienced when it comes to plastering, but I need a bit of advice please.

I have, here and there, removed some backboxes from the wall, filled large holes, filled large cracks (small cracks that i have since raked out to large cracks).

Basically i have used old multifinish for this, as i have read on here as the plaster isn't good enough to plaster with as it goes off straight away.

However, once filled (happy with finish, smooth etc) and painted, there are hairline cracks that take on the shape of the filled hole! I sort of assume that is down to me using the wrong filler, but it did also happen with some patching plaster that i used - so i then assume its just life... cracks happen etc...

So... I am removing the tiles from our bathroom, which has left some holes behind. I want to fill these obviously, I was going to use the multifinish i have to fill them. I will then be skimming the whole wall, so not too bothered about the fact that there may be cracks where i have filled it, but i am worried the cracks will then come through the newly skimmed surface.

Any advice on my best course of action, am i worried about nothing or should i just use the multifinish?

Thanks

 

[PrenticeBoyofDerry]

Finishing plaster is not ideal for patching/filling large voids. It is only supposed be used as a skim over of about 3mm.
You need to use a basecoat for filling large areas or you will get shrinkage and cracks.
On most surfaces especially dry dusty ones it would be advised to prime the area first to avoid shrinkage also.
Old multi finish is most probably the worst product you could have used.

 

[EdTurtle]

I should probably mention that i said i used multifinish to fill old backboxes... what i meant to say was that i filled the back box with concrete/mortar first, then literally just used the multifinish to top up the last couple of mm, sorry i did suggest above that i just used the multifinish...

But I'm wondering why i have read here and other places that they say to use old multifinish thats "gone off" to fill chases etc... if you say that its not suitable for larger voids?

So, do you recommend that I dont use the multifinish to fill the slightly deeper holes left where i have pulled the tiles off then?

If not, what should i use, as a basecoat? also, i have 'painted' the holes/voids with watered down PVA, is this ok? Walls are thermalite then rendered with concrete, then skimmed (can take a picture of the layers if needed?)

Thanks again

 

[PrenticeBoyofDerry]

If you have multi-finish that is past it's sell by/ best used date. Then I would much rather plaster/skim small areas, as it will go off quickly, but still normally usable for small areas.

Whether the multi-finish old or new, it is not a product that should be used for deep areas. It is not designed for that, it will crack and break, due to the curing and shrinkage. Even if you mixed up with a basecoat it will suck all the moisture out of the mix.
Unsure who has recommended or the purpose. But chases are better filled by a recognised base coat or even dry-wall adhesive.