Not a cure for rusty arches, but if the decision is made to change a rusty car, and a mint low mileage one is found with no rust in the rear arches (not many of those about now, I admit, but...), then the following may be of help. I have done it on all my Omegas since I bought my first one in 2004, and not one of my current ones (I have five at present 😀) including the first one I bought in 2004 have rusty arches... I first did this experimenting with ways to stop the wheelarch rust on Vauxhall Cavaliers many years ago, as they suffered in the same way as the Omega.....
Remove all interior trim from the boot /tailgate area, including fuel cap motor, and the radio electric gubbins in the rear n/s corner on the Elite estate, so that you can gain as much access as possible to the inner arches. Rig up a small light so that it can pass through the upper holes around the wheelarch so you see where you are aiming. Use a thinish rust proofing compound such as Supertrol (very good at penetrating welded seams) or a Bilt Hamber product with a long flexible lance. Use a small hand held garden spray for the Supertrol product initially set on a jet rather than a fan spray (you will need the accuracy and distance of the jet to get right to the front of the wheelarch), but make sure you fully extract each rear seat belt before starting this and secure with a peg, or you will end up with the fluid all over the exposed part of the seat belt! Make sure the lance is correctly in the area around the lower wheelarch, and the start applying. Do it on a warm day, and you should see the fluid eventually start seeping from areas you never imagined, but at least you can see where it's gone and that it's going to protect your wheel arches from the dreaded tinworm. It won't stop rust that's already in there - it may slow it a little, but remember that if you plan on welding any already rusty areas, then the compound will ignite very easily! I would only recommend this for known immaculate arches, or ones that have been repaired properly with new metal.
I have no connection to these products; they are ones I've used and have liked. Waxoyl is ok, but you have to thin it right down with white spirit for it to seep and penetrate, otherwise it bridges seams.
Like I say, I have done it on all mine - they all stand out all year round, and none have got any rust whatsoever in the arches. Well worth half a days work one sunny afternoon, I would argue!