Hello, Skody, no it's not trough, rust is only on top, steel inside is good. Wheel brake disc was in rust too, but I change it. If you do not drive mazda for some time, rust is coming very fast on brake disc :) On arches rust is too only on top, rust on arches coming because before there was R17 225/45 rims, and when some one siting in a back seat, tyres damaging arches on fast driving.
What's happening when You place R17 225/45 rims on non sport mazda 6 (in this photo R16 195/55):
Mazda 6 arches is made from very bad material, inside of the arche is some material like wool,so that think all the time sucking water inside of him and making mazda 6 arches to get rusty very fast.
One more gift from mazda 6: