As an Ami owner, you will become familiar with rustproofing, cos if you don't you won't have an Ami for long.

Cavity waxing is the coating of the insides of cavities including box sections with a film of wax. The reasons why you do this is because inside these areas very little paint or protection of the metalwork and mositure finds its way inside causing rust to work from the insides out.

On an Ami you should be waxing the following areas

Roof box sections above the doors.
A posts
B posts
C posts
D posts
Above the windscreen
The box section below the windscreen
Sills
Lower bulkhead box section
Top hat sections on floor
Jacking points on floor
The channel retaining the sealing rubber on the sills
Front inner wings
In the seams on the inner rear wings around the rear bumpstops.
Inside the box section at the back of the rear wings.
All seams on the rear wings (unless you have polyester type fitted)
Tops of doors underneath sliding channel.
Inside the top of the door and along all seams
Inside the rear seat box.
The rear chassis beam that supports the boot floor and inside the chassis.

I think that should keep you busy, but it will save your Ami from becoming a rusty stain on the concrete of your driveway.

I think what hes getting at is do you /how you just get a coating on the areas inside the cavities ,or how do you know its doing it rather than actually filling up the cavity with waxoyl or just the bottom part where its settled.
I noticed i'd completely filled my rear door box sections on my van when i picked the paint away that was holding the metal skin on and i could see inside the box channel i'd filled it right up
One other thing was pointed out to me once, with waxoyl blocking up the vent holes of cavities and box sections how does the area 'breath' in other words surely thats worse for condensation accumulating

Ideally if yuo use a high pressure cavity wax gun and a compressor this will atomise the wax and cover everything but not fill it.

Most companys wax products claim to halt rust in its tracks. So if it already has some corrosion a good waxing 'should' save it if done in time. Im not sure it ever really stops it, but certainly slows it down to manageable levels in my experience.

My preffered product is dinitrol these days but plenty use waxoyl or bilthamber etc.