The entire underside of my car is covered in paint, and then stone chip. Only the exhaust tunnel is not. There is no external way for the sills to rust if that stone chip is intact. If you have rust on the jacking points, or rust arches and water has got into the sills, then you have a problem.
But in the 14 or so williams i have owned, i have never had a single one with any rust on the sills. So to say it is common is misleading at best and poor advice at worse.
Are you confusing sills with INNER sills???
I've had 4 Williams' and only one of them had rusty inner sills. The other three had scabby arches as per usual.
I've had around 8 valvers and apart from my current one, all had rusty inner sills. Granted some were not 'that' major, but two were proper bad.
I thought that's what the op wanted to know, what were common faults on mk1 clios? Rust is the biggest one. If you deny that then you're a pellet.
Maybe I'm lying for some ultimate forum cool points? Or maybe I'm passing on my advice based on the cars I've owned. Who knows!
What about the red one that stu has posted on the previous page?! That whole back Wing is full of rust and filler! Look at the state of it!
Either way, I've decided that you're a chopper, so I'm now out.