It sounds like plastering that I might have done.....
Use an orbital sander, lightly, using 100 or 80 grit paper. Much more comfortable if the machine has a vacuum dust take off!
John
Finishing plaster should not be sanded; it’s not designed for it, destroys the grain & will look like suede brushed the wrong way when painted requiring many coats to cover & even then it’s still likely to look like shoite. You’d do much better to fill out the trowel marks with Easy Fill & sand that.
Who did the plastering? They either need a lot more practice or you need to change your plasterer.
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