Many thanks for the great site! Apologies for the lengthy text.
I have a joining wall in a downstairs hallway in 1910ish terraced property. I intended to remove the paper and skim but some of the plaster had blown. I have removed the blown stuff and I reckon it accounts for 40% of the 2.4m tall by about 5m long run of the hall.
Now the brickwork itself is irregular and I can see from inspection that the thickness of the original backing plaster varies between about 10mm all the way to 25mm in places to account for this.
I am considering taking it all off rather than patching but 5mx2.5m seems a bit much to solid plaster for a rank amateur such as myself (my limited experience is in skimming and patching).
In my eyes the benefit to patching is I have parts of the wall existing that I can rule to. However, it sort of feels wrong to do this. So, finally, on to the question/s.
Is it a good idea to make a series of guide bars either horizontally or vertically in hardwall and let them go off with the intention to then fill and rule off (again in hardwall) in sections? Are there any problems with joining set hardwall to wet hardwall?
Firstly don't worry about patching seeming wrong - it really isn't. Professional plasterer's will make the judgement on whether to patch or re-plaster the whole wall based purely on cost (time and materials). We play to our strengths - often bigger areas being more efficient, whereas small patches can be time consuming.
However you need to play to your strengths and practicalities - time is perhaps less of an issue, and as you say to your abilities - why not indeed make the most of ruling to the existing plaster.
Perhaps an equally important consideration is the skimming. Patching is very time consuming and often not as good a finish can be achieved as when skimming the wall in it's entirety.
Patch backing - skim whole wall?
So one way forward may be to patch the backing coat and then skim the whole wall (treating patch and existing as one whole backing coat)
The only problem with this is you may have different suctions (porosity) which affects how quickly the plaster will firm up. For a good finish this needs to be as even as possible - which can be achieved by applying two coats of pva across the whole surface. One coat will ensure adhesion between the old plaster - the 2nd will reduce suction (porosity). If when applying the 2nd coat you felt it was drying very quickly then a 3rd coat could be applied.
However if you did decide to remove all existing plaster and use guides as you suggest in your question: the problem with joining to set hardwall, is that you need to be so very thorough and ensure that the guide is so flat with no bumps. Plus it can be difficult to get the guides flat in both planes (horizontal and vertical). It's for this reason I suggest using sand/cement as you have the whole day to work with it. Your guide may have firmed up due to moisture being drawn out of it but it will not have set, and so if when filling in between the guides if there is a bump or the guide is not flat in the vertical plane - you can simply shave a bit of with every pass of your straight edge or add a bit to it as needed.
I hope that all makes sense! It's a tricky subject to explain but once you've watched the backing coat videos you may well understand what I'm saying if you don't already!
But the best answer is to do what you feel will give you the best result - as you know what you can and can't do, plus if you choose the method you enjoy the thought of - you'll be more enthusiastic to obtain a great result!
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