hehehehe ;D
most overskim is wobbly mate... youll get some out with the skim, but not a lot... only the sharper dips and hollows but you know that already...
i always explain it too em, if they want it a bit flatter ill charge em for the bonding out, if they want it dead flat its near enough a full wall float and set..
most people (99 percent) would be more than happy to have the worst of it taken out then skimmed nice finish, when its painted it looks a million times better than the origional...
if you stick a feather edge over most walls over 20 years ond theyre never flat...