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This is a 30 year old wall. The old plaster is removed. Beneath the old plaster there is bumpy mortar left. What is the best way to get rid of them and get a nice flat surface ready for replastering? enter image description here

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I know this is not the answer you are looking for, for this wall is ready for plastering as it is. It will need a scrubbing or power washing to remove all or most of the dust left over from the original plaster.

To get the wall flat like you ask Will require at least scraping with a tool like this. (image courtesy Razorback)enter image description here

This will be very time consuming and if the mortar is very strong, it may not do what you want. The most reliable way to flatten the wall is with a right angle grinderenter image description here and a blade (disk)enter image description here (Images courtesy Amazon) made for cleaning the surface. It will be a very, very, dusty job. You will need to wear a protective dust mask and face sheild.

The art of plastering will require a build up coat and a final layer, if you are planning on true plaster. There are newer versions of plaster look a likes out there, but most start with a base layer to prepare the wall for the finish coat. This eliminates the need for getting the wall perfectly flat.

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  • Note that dust can be explosive and take precautions. The mortar shouldn't be a problem but other sources might, including old paint and dust that has settled over time. ccohs.ca/oshanswers/chemicals/combustible_dust.html
    – Ack
    Mar 13, 2020 at 16:29
  • Yes, that is true, but the dust has to be airborne if it is present. If there is dust on the floor that could become airborne, vacuuming it up will solve that potential problem. The haze I see remaining on the brick looks like the residue left form the original plasters' contact to the wall. Of that, what does not come off by power washing or scrubbing can be coated over by the new finish.
    – Jack
    Mar 13, 2020 at 16:49
  • correct that it's airborne dust that is the concern, not dust in itself; I neglected to add 'airborne' in my note, thank you
    – Ack
    Mar 13, 2020 at 16:54
  • a scraper chisel blade in a rotary hammer drill could be another option.
    – Jasen
    Mar 13, 2020 at 23:41
  • Yes I did consider that as part of the answer. For a DIY the tool selection can get pretty elaborate, so I kept it to a few basic tools, Even the right angle grinder and the grinding wheel do not come cheaply.... Yes a rotary hammer set to just hammer and the spade bit attachment will work as well, but I can see being pelleted by chunks of mortar all day, and being wore out at the end of the day.
    – Jack
    Mar 14, 2020 at 0:07

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