Weathering Steel never completely stops rusting. The weathering process occurs after the panel is installed. It eventually decreases and from that point on the finish acts as a protective coating for the base metal.
This protective coating essentially stops the corrosion. Laboratory analysis of the rust film have shown that the alloying elements in the steel, particularly copper, chromium, and nickel produced insoluble compounds that clogged the pores at the rust/steel interface, thereby ending the regeneration.