How do you remove rust from metal door?

Sorry, but personally I do not have much experience with this issue. However, I have found some information for you.

According to HammerZone.com, you most likely have to replace the door. There are some cheaper alternatives to buying a pre-hung door unit.

This is quoted from HammerZone.com:

1. Buy a nice solid wood door and cut the hinge pockets with a router. This is tricky, I've never done it on a replacement door. A good carpenter should be able to do this in a few hours, so even if it cost you $100 to $150 in labor it might be worth it.

2. Investigate fiberglass doors. I've never worked with them, but they are gaining in popularity. And they don't rust, won't rot, and some can be stained with wood-tone colors. I'd ask around... and look beyond Home Depot and Lowes, ask at your local lumberyards.

3. Repair the existing door. I would try this first because it is cheapest and you can do it yourself. It's best to remove the door by taking out the hinge pins, though, so look for a nice day (or cover the entry with a tarp or plywood). Lay the door on a pair of sawhorses in the garage. Put old rags on the sawhorses to prevent them from scratching the exterior paint. That loose strip on the bottom is just a door sweep. Some doors have replaceable sweeps. Maybe yours is just ripped and needs to be replaced. But you can always buy a sweep that is screwed to the bottom of the inside of the door.