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The problem is that a chisel can't "cut" this plaster - it can make it crack, and then come away from the wall in sheets, but it can't cut a neat channel in it.
That is why you weaken it by making "pilot" holes along the lines which you wish to cut. I had concrete on the wall where I fitted my new fireplace, and I chased a cavity through the whole lot by drilling and chiseling.
Difficult to comment when you can't see the task in question though. You may well need a cutter as you suggest. But I still don't know how you'll knock the plaster away from the brickwork with just a cutter.
rodabod-
That is why you weaken it by making "pilot" holes along the lines which you wish to cut.
"Stitch drilling" apparently.
If you use the cutter with a narrow blade you could effectively stitch drill with that as they're good at making plunge cuts. You may still need a chisel to get the plaster away from the brickwork though.
ffm
The problem is that a chisel can't "cut" this plaster - it can make it crack, and then come away from the wall in sheets, but it can't cut a neat channel in it.
It's just too tough.
Same with the SDS - which I've used extensively as a power-chisel, it'll cut into the plaster, but it will then crack and fragment into sheets, rather than allowing a neat channel to be formed.
It needs to be cut - angle grinder with masonry wheel works well here, but for the aforementioned dust issue.
I'll pick up a multi-cutter and tile/masonry blade, has anyone got personal experience of a good one that they can recommend?