My Discovery has a couple of very tiney rust specks on the exterior. I thought the car was made out of aluminum and therefore did not rust. Why is there rust:

 

What year and exactly where is the rust?

 

If it is a DI then all the body skins execpt the roof are aluminum, if it is a DII then the door skins and roof are steel.
Window frames are steel on all trucks.

 

The discovery is a 2000. So, the whole care is steel???

 

No. Everything is aluminum EXCEPT the doors and the roof.
Take a magnet off the fridge and walk around your truck sticking the magnet to the truck, this will help you under stand I think.
It can be confusing. Where is your rust?

 

The tiny rust spots are on the door and the roof. Exactly like you said. Spike- Let me ask you about underbody rust- The underbody has minor surface rust. I figured I would brush it off, prime it and paint it. Does this sound right? What does ruberrizing mean? Also, I have this spray called Permatex rust treatment- it says to simple spray on and then paint over. What is the best way of getting rid of rust and protecting it?

 

For underbody rust, just take a wire brush to it and paint it with Rust-o-leum.
Rubberizing is like a spray on bed liner for a pick up truck. I would do the insides of the wheel wells and anywhere else that you want, just not the frame.
the frame has holes in it to let water drain out. If those holes are plugged and the condensation cant excape then the frame will rust from the inside out. It will take 30 years, 40 if you dont plug the holes.