I ride a 1972 steel frame bike year round through Canadian winters.
When I got the bike it was in good shape, but after putting a few thousand km worth of commuting on it, it has a lot of scratches in the paint - mostly from things like road salt getting kicked up at speed.
In the course of the last winter, despite my best efforts, many of these spots started to rust. I can remove most of the rust spots with careful application of steel wool, but I'm now a little worried about the structural integrity of the bike. I notice some creaking when I shift more of my weight to the bars for instance.
How can I tell whether a bike with some rust damage is still safe to ride on?