Having own 3 previous jazzes, and now owning a Japanese imported ‘fit’ I know all about the rust problem. I live in Scotland where the roads are salted 6 months a year. I coated my last jazz (mk1) with waxoyl but I didn’t remove the rear arch liner first. It rotted out under the rear sill end behind the liner. When I bought my mk2 fit I noticed that it did not have a full arch liner. I removed the small one at the front of the arch and the rear one, and sprayed Dinitrol right round the arch. The whole underside has been sprayed including all the suspension, the engine bay and front arches all with Dinitrol. After the winter I will redo any areas where is worn off. The undercoating on all Honda Jazz/fit is minimal and very poor compared to other manufacturers. The Japanese cars are no worse, as I’ve compared them side by side. They all need a coating. Without it in Scotland they will rot through in 13 years no problem. A particular bad area is the area around the petrol filler pipe. It’s completely exposed to the salt. I cleaned it all out and sprayed all round the filler. Inevitably salt will build up round it so it will need hosed out every spring when I switch the wheels back to summer tyres. Worth the hassle to avoid the rust.