Do I need to prepare the surface first?

Yes ... preparation is always the key to a good job so you just have to do the hard yards. I use the large brush to apply sugar soap ... don't use it to scrub with ... then I find an old towel the best thing to wipe it off with ... that little bit of abrasion leaves the surface squeaky clean. In fact I keep some of my old pads for this very thing. Any loose flaky paint of course has to be sanded back ... standard procedures still apply ... if in doubt ask at your local hardware store.