Ron, Queensland has a frame inspection at 10 years, then every 5 after that, and I think once it gets to 30 it's not able to be used as a bus, this is all from memory, so I might not have that quite right. This was brought in about 20 odd years ago after a bus carrying school kids went over the Kuranda range, they all rust right under the windows, and the whole roof parted company on this bus. The newer bus bodies are made with duragal, then the side sheets and windows are glued in which eliminates a loy of water ingress. I used to do a bit of work for a bloke that does a lot of bus repairs, every rivit hole would quite often be rusty.
