I was Trek's warranty inspector back when they made lugged steel and bonded aluminum frames. Rust under the paint is obvious from the bubbling, and not a warranty issue. Even rusty steel frames seldom fail suddenly or catastrophically, and other steel failures e.g. from insufficient braze penetration or overheating of joints also typically exhibit slow failure modes. Our most common steel frame failure was broken rear dropouts; Shimano in particular had a run of problematic UF dropouts that necessitated mid-production substitution of Campagnolo dropouts. Aluminum has a very different failure progress than steel, and can go from an initial crack to complete failure surprisingly quickly. We replaced a significant fraction of the initial production run of bonded aluminum frames when cracks appeared at the tube/lug junction. destructive analysis of these frames eventually showed that this was not a joint failure, but an artifact of the paint being less flexible than the epoxy holding the joint together, with the result that the paint would crack but the underlying joint remained completely intact. Reformulation of the paint addressed this issue.