I'm preparing to skim a room and I've noticed that some of the old plaster has a hollow sound when I tap on it in places like it isn't bonded well to the brick some of the worse of it is pealing off the wall where chasing for cables has occurred.
Any advise would be great.
This is a tricky one to answer as there is not always a right or wrong answer - it is more what you are happy with.
It can be that hollow sounding plaster is absolutely nothing to worry about, as patches can lose their bond and be hollow very soon after application. If these areas are not that great an area, and anchored every now and again by adhered plaster, then there is often no problem. It can be surprising just how many walls have hollow sounding patches in them and will probably be there for 100 years plus. It is often only when doing building work that we become aware of them.
However if it is something that is going to worry you then consider patching them.
The hollow sounding areas around cables - this may be due to the patching plaster not adhering properly - but there is also a possibility that it is where the plaster is very thin over the conduit - which is totally normal and not a problem.
How I judge it
If the plaster moves when pushed I patch up. If I can detect no movement then I do not worry, unless the hollow area is vast and there is significant cracking. It is worth keeping in mind that the new plaster you apply will strengthen the existing - particularly if some scrim tape is pushed into the 1st coat as you cover any bad areas.
You could ask 5 plasterers this question and you'll probably get 5 totally different answers!
Hope this helps you decide
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