Just to add to this, you may want to paint with bin shellac primer to minimize any issues the mud may do to whatever is on the wall. Also, if your doing skim coating first time and want a dead flat finish, I would get the 45 min, 60 minute mud and do the thinnest coats possible. I’m talking less then 1/16” thin or pull it pretty tight so there’s almost barely any on the wall. You can scape any bumps and go again once dried. Skim coating is an art that pros who are good can get away with 2 to 3 coats or less. For people just doing it the first time, do 5 to 10 coats and do a 10 to 12” trowel and put the mud on the trowel, don’t paint it with a roller. It’s much easier to control but will take a lot of time which is why you should do the 60 min as you can go again once dry. You’ll have some divots from the old wall not being flat and parts that are uneven but each time you will fill it in until your last coat is flat and smooth. If you do thicker, you risk having uneven spots like you have. Some people like it but that is why your walls looks that way because folks do 1/4” or more of mud/plaster and then sand high spots but it’s left with larger uneven spots that looks similar to stucco.