I used a cheap endoscope to look inside my sills accessing them through the grommets in the rear wheel arch.
I don’t have any signs of rust externally. The photos are shown in sequence starting from the furthest in somewhere around the door opening and working backwards.

You can see there is some wax in there but it’s not well dispersed and hasn’t stopped rust starting right at the back.


About 1 metre inside sill from rear wheel arch


About 300mm in from rear


Immediately inside grommet

 

Hi, is that the left side,
The bottom you see is the inner sill, under that is the outer sill, the rust if any will hide in there until it show's through on the outside.(or in)
The front half dosnt have that extra inner sill, you can look in from the front wheel area (same access grommet there also)
Under the door you can see a brake line, this is where the extra inner sill starts and continues back to the rear wheel.

 

Yes it is the left side. I looked in the right side as well and it was similar.
is there a way to get cavity wax between the inner and outer sill?

My car has Paragon bumpers and side sills, I think the plastic sills must be bonded on but I’ve not seen a standard car to compare it to. When rust does get going in the sills it will be bad before it starts to show.

 

Not that i know...(yet)
You could drill a small hole half way between the brake in the front/back sill from under the sill and plug it after, about 300/400mm from back wheel..

 

It is possible, on the coupe at least, once the interior side trims are removed (think rear seat out, seatbelt harnesses dismantled).
The inner and outer wheel arches are bonded rather than spot welded. The bonding lets go, and in goes the water. I ran a thick bead of hot waxoyl down the wheelarch:



However, there is a strip of sealing foam running lengthways part way down the sill cavity. This pic. is of the front of the wheelarch dropping toward the sill.



View forward toward the join below the door:


I have no idea how this was done during manufacture.

I punctured this foam with some hard tubing to get to the bottom where the layers meet at the bottom seam. Put lots of your favourite ant-rust in there. It wil run along the sill and exit wherever the seam sealer has also failed on that bottom pinch joint.

Pay attention to the inner/outer wheelarch seam where the problem starts. I have given the wheelarch lip several coats of waxoyl to ensure the seam is completely sealed from the outside.

By the time you see paint bubbling on the outside, the damage is already done.

 

Well I took it for rust treatment today at Krown Rustproofing near Tipton (UK). Krown is a big chain of companies in the US but over it’s just one guy using the Canadian products.
I talked to him about the potential rust spots I’d found out about, he knew them all and more as well as how to get to treat them.
In 3 hrs he had the wheels off, wheel arch liners out, sill covers off, rear lights out and boot trim. Then he injected in to all the cavities, the doors, the boot lid, the bonnet, fuel and brake lines, suspension, sub-frames, floor pan, engine bay, radiators, cross members, if you can name it, he treated it.

I got to look all around it before he put it back together and it was in great condition. The best he’d seen for a very long time. So I’m happy.