give it a good sand prime it with kilz or bin 123 or bullseye 123.and texture it you will never get it perfect
thin coat or skip trowel it.
get some 1/4″ drywall and cover it.
For about 12 and let it and mud for about 12 and paint and paint that high with latex paint and ceilings easily those over mud those too lowes and mud for about 12 and give you perfect job.
skim coat it with gold bond lightweight top coating ready mixed joint compound.
If its painted smooth and not textured just dab a little nail hole filler and smooth it with your finger or putty knife.