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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?

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  • Where are you in the world? Have you any reason to suspect that you've got lead-based paint? (Sanding/grinding that is bad.) Do you have the option to shorten the blind and fill in the material below (as opposed to removing material at the top)? – Aloysius Defenestrate Jan 18 '20 at 14:27
  • One last question: how far into the window opening is the track set? (ie, is it flush with the wall, or can it be pushed in.) If it's inset a bit, you might get away with shimming the track plumb and applying a bead of caulk. – Aloysius Defenestrate Jan 18 '20 at 14:31
  • I'm in the UK and the building is 1930s, so I gather it's quite likely to have at least had lead paint applied at some point in its history. Re shorter blind: though I already have the blind, it might be possible, though awkward. But if adding plaster instead of removing, would the paint not first be removed anyway? (and more of it, because a longer length) – Croad Langshan Jan 18 '20 at 15:20
  • The blind is inset into the opening, but it would be about 1mm or so too wide without removing this material! – Croad Langshan Jan 18 '20 at 15:21
  • 1/10 of an inch? Then you measured wrong for the blinds (did not subtract 1/8"~1/4"). - "so a gap is not left between the battens and the wall" - that's what caulk is for. – Mazura Jan 18 '20 at 18:11
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I would forget about power tools and get a 1" chisel. Trace out the area with pencil and shave out the area with the chisel. You're talking about a very small area so it shouldn't take long. Afterwards, smooth out with a sanding block. As always, wear a dust mask when sanding.

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  • Interesting, that was my first thought, but I dismissed it as likely bad for both the plaster (flaking / crumbling?) and the chisel (blunting?). I'm certainly tempted to try it though! – Croad Langshan Jan 18 '20 at 16:48
  • This is plaster. It will shave nicely and you won't dull the chisel... Obviously, you won't need a hammer. Again, you're talking about a really small area. – JACK Jan 18 '20 at 17:37
  • @JACK, if you don't mid me adding, If the chisel is used with the flat back against the corner bead, and the bevel of the sharp edge out, the flat back will help gauge the angle and depth of cut much easier. Starting at the corner bead, and the sharp edge of the chisel at the corner bead, push the chisel in at an angle upwards to clear the plaster out of the way. Definitely no hammer needed, but care must be used. – Jack Jan 18 '20 at 19:35

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