well my 93 car has zero rust on its sills, it has the bubling effect thing behind the trim covers as most do but u can hardly tell, like houston said just buy new ones for around 30 quid a side if its doing your head in.i personally aint fussed about it.
i painted my sills (the bits that stick downwards under the skirt along the chassis) with black satin hammerite just to protect them for the rest of its life, but i was very pleased their was no rust their at all, infact the underside of the car is perfect

, i think it all depends on how hard of life the car has had and if they have been kept free from crap that holds moisture and wet.those plastic skirts dont help as they hold crap behind them...
another thing, always make sure u wash and remove any muck from under the arches as a build up their will also eventually help cause rust/rot