Prevent door bottoms from rotting or rusting by installing a drip edge.
A standard door bottom drip edge can easily be cut and attached to the bottom of a door, blending in after caulking and painting. Most exposed doors rot or rust from the bottom because the bottom edge cannot be protected. The drip edge keeps the water from curling under the door, instead allowing it to drip onto the threshold.
To repair a rusty door, sand the loose rust off, clean and spot prime the rusty area with a rustoleum type primer. If there are rusty holes, apply Bondo paste after the primer, then sand and prime the Bondo.
To paint, go to a good paint store and purchase acrylic metal paint and apply it in the cool of the day with a good brush.
Best, Charles