Patching and Plastering - bad vinyl patches by previous owner - how to smooth?

The 2 walls on either side of the fireplace have bad thick vinyl spackle patches. The patches are 4"x4" approx, and irregular shapes.

I can tell they tried to quickly spackle over some drywall anchors from prior shelving.

There must have been 3 or 4 shelves on each wall, because both sides of the walls have these giant ugly patches that the prior owners never sanded smooth.

So I have to fix both sides, top to bottom, floor to ceiling (8ft height).

needless to say, it looks awful.

My question is: should i sand them down which will leave a deeper defect b/c the stuff comes off like a thick skin?

Or, should i try to skim coat over them, floor to ceiling, without sanding over them?

In other words, do I just build it up? or do i 'break' it down before i build it up?

thanks in advance.

It's sounds pretty bad. I wouldn't try to sand it. There is a third choice. Cut out the bad sections & install new sheetrock.

If there are any high spots, they should be knocked down. IMO it's easier to scrape them than to sand. You will then need to skim coat the area until smooth.

almost forgot:(
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