Lots of the time people think their car is rust free because they can't see any on the outside.

I was one, having owned my UK car since it was 8 years old.

I discovered rust having owned the car for 10 years and always being highly attentive to it.

You need to look into the rear of the sills

(the only way is to strip back the panel behind the seats and remove the foam lump from inside the skin construction

- chances are that if there is foam there is rust in the sills unless the car is a later import and or been garaged).

The other place is behind the rear wheel wells accessible from behind the boot carpet.

I have paid out a total of 2k for a proper job repair to the 2 sill ends.

Anyone who says they have paid less than

£500

(per side) has only has the sill covered over with fresh metal, trapping the rot in place once again.

The internal structure 99% of the time needs replacing as the aforementioned foam traps moisture and rots the sills from inside out when it drops onto the sill structural boxes inside and blocks the drainage hole.
