Please help- finishing coat blown badly

[Graym0]

Last year we installed a Newton system in the cellar and used a cement based render over the waterproof membrane covering the walls.

We followed the instructions on the Newton site using six parts clean sharp sand/one part lime/one part cement. A two-coat application was applied leaving plenty of time for drying. This was then skimmed with a 3-5mm finishing coat, and painted.

I recently noticed a large crack down one end of the room where the finishing plaster has literally come off the wall. I started to peel this off hoping I would get to a stage where the edges were all bonded to the render but I now have a large area of bare render(~600mm by 300mm) and some of the edges are still loose, moving when I put pressure on them. I'm worried that if I keep pulling more off I'll end up stripping the whole wall.

I didn't do the rendering/plastering (I'm not a plasterer) but was thinking that I could try to use something to re-bond the loose edges, add a couple of coats of PVA and then skim the area with finishing plaster. Any help on what to do here would be really appreciated.

Thanks in advance,
Nick

 

[roy c]

Is the rendered wall flat with no "scratches on it,(Where the plaster has come off)?

 

[Graym0]

I've just uploaded a photo here..

//www.diynot.com/network/Graym0/albums/

 

[roy c]

The wall should of been "devilled " up before it was skimmed and it also looks like the skim coat is way too thick. It is custom to lay a first coat on tight and let it pick up and then lay the second coat on and should only need to be a few mill thick finished...might be best to scrape all of the skim coat off and devil it up (or get some sbr on it and get it skimmed properly...

 

[Graym0]

Thanks Roy.

The skim coat is actually 3mm (inc. paint) so I think it's probably just the photo making it look thicker.

When you say Devil it up do you mean cut deep scratches in the surface? I know this was done between the coats of render. The surface is quite rough to touch but doesn't have scratch 'channels' cut into it.

I was hoping you weren't going to say pull all the finishing coat off. It seems to be bonded well everywhere else. It's just this small area and most of the edges are bonded - just one side is a bit dodge so was thinking I might be able to use some kind of adhesive to push under it to firm it up, then just plaster this small area?

 

[stevethespreader]

Don't bother pushing stuff under it cut back the skim till all is firm then patch it , what under coat is it? Its a misconception that all undercoats have to be scratched, with gypsum undercoats if left off the rule that should provide enough key

 

[Graym0]

Thanks Steve.

The undercoat is two layers of cement based render- six parts sharp sand/one part lime/one part cement mix.

It's easier said than done to keep chipping off until firm, because when I lift a bit, it seems to loosen more- perhaps cutting it would be better so its not lifting other areas.

 

[stevethespreader]

You could angle grind the edges so it don't chip more off than you want (dusty though) or do as roy says take it all off,s&c render should be scratched