How do I fix a bad plaster job so I can repaint and it won’t show?

Sand it smooth then re plaster as smooth as you could till it’s level

Use a primer if you are going to paint. ☺️

If you have cracks in plaster, the only way to keep them from coming back is to use the mesh tape and spackle over it. You may need to "sand, spackle, sand" more than once to get a smooth feathered edge. If the wall is a total do-over they sell mesh that is on a roll that is three feet wide. You press it on to the wall and skim coat over it. It is very easy to work with and can save a great deal of time, especially if your plaster wall has a lot of little cracks. If you have the time and the inclination you can use a box cutter or blade and remove the paint around the cracks before sticking up the mesh tape. By creating a trough for the tape to sit in you can have a less prominent repair. You may want to avoid glossy paint finishes if you are less than meticulous in your sanding efforts.

Have a drywall contractor come in and texturize your wall. I've flipped a few houses and managed to hide a multitude of sins. It's a simple and pretty inexpensive process but best left to a pro so you get a uniform and consistent result. Always ask to see pictures of their work so you are both on the same page. Lots of textures to choose from. I like Spanish drag and stucco finishes. Always prime after and then just paint. I recently did a fairly large living room and it cost about $175. Only took a couple of hours. Price didn't include priming or paint.

Google "pictures of drywall texture"