Rust, Rot and Moisture Damage on the Garage Door in Allentown, PA

What you are seeing

Rust spots, streaks or bubbling paint along the bottom panel and the bottom edge of the door.
Chalky white corrosion on an aluminium door, or a crust of salt and grit along the bottom section.
Soft, spongy or crumbling wood on a wooden door, especially at the bottom rail and the panel edges.
Peeling or flaking paint, or a finish that has gone dull and chalky all over.
A door that swells and sticks in damp weather and frees up again when it dries out.
Water pooling at the door apron, or a bottom seal that no longer meets the floor.

Inspects the door panel by panel from the bottom up, and probes the bottom rail and panel edges on a wooden door to find how deep the rust or rot actually runs.
Checks the bottom seal, the apron and the drainage around the door, because a repair that leaves the water source in place will fail in the same spot again.
Sands, treats and refinishes light rust and surface rot, or replaces the affected panel, bottom section or bottom rail where the damage has become structural.
Checks whether corrosion has reached the bottom bracket, hinges, cables or springs, since a corroded bracket under load is a safety problem as well as a cosmetic one.
Tells you plainly whether the door is worth repairing or is far enough gone that replacement is the better value.

The door make and approximate age, and what the door is made of - steel, aluminium, wood or a composite.
Photos of the damage: the bottom panel from outside, the bottom edge from inside, and any rust or rot at the edges and joints.
Whether the door sticks, drags or is hard to lift when it is damp or after rain.
How the door is finished and when it was last painted or stained, if you know.

A door with rot at the bottom rail, or rust through a hinge or bracket, is weaker than it looks and can fail while it is moving. Keep clear of a damaged door and do not lean on it or lift it to test it.
Never stand under, or reach under, a door you are inspecting, and never probe a wooden panel with a tool aimed toward your own hand. Work from the side and treat the opening as live.
Do not try to remove a panel or the bottom section yourself to look behind it. The cables, drums and springs are still connected to that section, and the door is not safe until they have been released properly.

Is this damage cosmetic or structural, and how did you decide?
Can the affected panel or bottom section be replaced on its own, or does the whole door need replacing?
What is the actual source of the water, and what will stop it coming back?
If you repair this, what condition is the rest of the door in, and how long should the repair last?
Would you repair this door on your own house, and if you would replace it, why?