Another of my dumb questions: I want to sand smooth some uneven/bumpy plaster prior to painting it. I have a belt sander and a back-and-forth finishing sander; the speed of the belt sander is fixed and I'm concerned it might be too aggressive but I think the finishing sander is maybe a bit weak; would it be better get a random orbital sander? What would you do (other than removing the plaster and having it redone properly; it's been like this for 30 years or more but now mum wants it fixed).