I sympathise with you Shady, on my old one the jack went right through the sill.
I had to wreck a pirrelli and drive it to a tyre fitter
My new one however is in the shop having the jacking points fixed and re-enforced.
I must say i bought the least rusty Mk3 i could find and I saw some rusty ones right across from 92 to 98 plates.
It seems purely luck of the draw for most of the normal rust, but most of the sill problems come from buggered jacking points and the damage resulting from falling off the jack.
I think if you own a Mk3 then you need a proper jack in the boot, makes a good anchor too