Salt air working under the seal
Wind off the bay leaves a fine salty film on cars, and over the years it gets into the edges of the windscreen seal. Where the paint around the frame has been scratched, rust starts under the urethane where nobody can see it.
Rust that is bonded over keeps spreading and the new screen leaks. The first sign is usually a damp carpet or a musty smell after heavy rain.
We inspect the frame while the glass is out: surface rust is treated and primed before bonding, and anything more serious is shown to you before we go further.