This is a photo of a mediocre/unfinished plaster patch job that I would like fixed.
The sheetrock patch is 1/2" thick, and the surrounding horsehair plaster is roughly 3/8" thick, I believe. The initial person tried to blend the edge a bit over the mesh tape, but didn't finish the job. The square in the photo is roughly 2' by 2'.
Ideally, you would cut out the patch and put in something the right thickness, rather than trying to blend the uneven thicknesses.
