Dean,
As Gavin said, Rust never sleeps.
The repair is dependent upon the extent of the rust damage. Usually if the rust is bubbing through the paint on the lower door, it probably has rusted through from the inside of the door.
Water gets in through the weather stripping around the window which is normal, so they put drain holes in the bottom of the doors to let the water drain out. If these holes get plugged up (they often do) you will have a puddle of water in the bottom of you door and that begins the rust process, so it is always important to check that the drain holes are clear.
This kind of damage will usually require replacing the metal at the bottom of the door. When you cut it out, you will be able to examine the extent of the rust damage on the inside of the door....if it is just surface rust, you can use a rust-converter product that will prevent/delay the rusting in that area.
If are lucky, and the rust is just surface rust caused by chipped paint at the bottom of the door, then you may be able to just sand/grind down to the bare metal to remove all the rust and then reprime and paint the door.
The easiest way to check which condition you have is to use a sharp object like a screw driver or knife to poke at the rust spots....if the blade goes through, the rust has perforated the metal and it needs to be cut out and replaced with new metal. If the blade does not go through, and feels solid, then you probably do not have a rust through condition and you only need to grind down to solid clean metal, prep, refinish and rerepaint the door.
...Rich
