This question has come up a couple times recently in my work. I have some clients who need stuff repainted that's currently painted with semigloss paint (latex). I know you need to "scuff up" the surface in order to get new paint to stick to it.
My question is, is it better to physically scuff the paint with sandpaper, or to chemically etch it (someone mentioned using TSP in another thread)? I suppose it may depend on the specific situation, right?
One thing I do know about TSP is that if you use it, you'd better make damn sure that you remove all of it before you paint, else the paint won't stick worth a damn.