I was figuring this. I grew up in Northern Michigan and did quite a few “Rust Repair” jobs in a shop that I worked in, and what you see on the surface is just the tip of the iceberg.
I’m planning on having to replace my inner panels, as the sill plate and Door Seal lip are showing rust.
I can’t see any rust from underneath, but I haven’t opened it up to see what’s inside. I’m hoping for the best case, and find that the inners are fine and I just need the the outer.