Mine rotted out there,rear cill, so did t'other side and the drivers forward cill
You might be lucky,but on pressing the blisters mine turned into a major rework of the cills.
I'm guessing I stumbled upon a water ingress trap,(at least on hatchbacks anyway..) :
Take the rear wheel off,or you could just set the car on high,with axle stands etc, so as not to chop yer arms off..
Look inside the rear wheelarch, to the left, there's a small 2 - 3" square plastic inspection cover kind of thing..mine had nearly become completely detached...letting water spray in to the cill, (I could see inside the cill with a torch, looked like a perfect rust pattern of water flow spraying in.
So after both cill rebuilds I blathered the insides with oil/grease/owt I could think of to protect it.
Then, I undersealed my cills an inch higher,which covered most of the repair,but can't be seen when the car's sat level again.
That lower paint hides a lot as well..it's like a stonechip/underseal plasticy stuff,painted to match the car. Like I say,you might be lucky, but on mine, pulling that thick paint off revealed mahoosive holes/rot!
Hope that's of some use..and not scared you to death

1999 Xantia LX 2.0 HDI 110 Rusteration project...