Rust prevention.

I have a new passanger side wing for my car, I now refuse to change it as I know the car will not look as good as yours

Rich.

that chassis behind the wing looks lovely. ^^ be proud ^^

Dynax S50 or Dinitrol are great undersealants - if you want some long-term protection on non-rusty underbody areas. Don't bother with waxoil, it's been superceded.

I've had one car S50'd - my blue low-mileage one. I've also found evidence on the white car of waxoil but it looks old and was stupidly applied,

I use ACF50 as an annual treatment (pre-winter prep, each year) - pretty much like bikers use it. I try and blast the arches, spray up inside the sills, behind the wings, on the insides of the wings, areas around the engine bay which give access to the chassis, etc.

ACF50 is much lighter than an underseal - it's much heavier than say, WD40 though, and actually effective - but it has some cool properties - it can wick excellently into rust, and evacuate moisture from that rust to the surface, so there's no water getting to the metal beneath. It lasts a while too, unlike WD40 which barely contains any actual oil.

Another option, if you have exposed rust.. aquasteel. It's a mix of wine acid (I forget the name - stinks of vinagre) and a rubber in liquid form, you clean up the rust you can get to (with a wire brush) and spray this on - it turns the red oxide into black oxide, and leaves a rubber-like coat above that.

I've used the raw form of the wine acid before and it is great for nuking surface rust off an existing rusty thing, the idea is to get the loose red oxide off and then treat it to go black - so there is just black oxide and metal beneath (it then acts like a primed piece) - you can then paint it over with something like hammerite, which if applied property will outlast the car.