I'm fitting a blackout blind and the width of the opening of the window recess at the top narrows over at the the final 4 cm or so (1.6") by a maximum of perhaps 2.5 mm (1/10 th of an inch). The bulge outwards of the plaster is bigger the closer to the top of the wall you get, so more needs sanding off at the top than the bottom of that 4 cm section -- the 2.5 mm or so is the most is the most it bulges out at the top. I'm not sure of the material under the plaster, but the building is of late 1930s concrete construction.
The blind runs in tracks that attach to thin battens that will be glued and nailed to the plaster.
So that the battens that the vertical blind tracks will sit in are straight and so a gap is not left between the battens and the wall, I need to make the vertical wall straight. So I need to remove a roughly wedge-shaped piece of plaster material of max 2.5 mm thickness over that top 4 cm of wall, with at least the same width as the batten, which is around 4.5 cm.
I certainly don't need it perfect but I'd like to make the finished job look reasonably neat.
I'm assuming the best way to do this will be by sanding using an oscillating tool. Am I correct, and if so what's the best way to go about it? For example, what sort of attachment (shape/material etc.) would be best suited, how can I achieve the straight edge I'm trying to get, and do I risk losing big chunks of plaster and giving myself a bigger job?