Why did rust appear after my first layer of seasoning?
Are you sure it is rust and not oil residue? Is it sticky?

In your case, I would try to clean the pan by heating it on the stove with some water in it. Use low to medium heat. Once it's warm, use some balled up tinfoil to agitate the surface of the pan. If it's really caked on, drain the water and try repeating the process with some beer or liquor as a solvent..that will usually do the trick. Wipe down with damp or dry paper towels to remove residue. Repeat steps until clean.

Once clean, I typically air dry or heat the pan back up on the stove on low heat to get rid of any residual moisture. While it is still warm, lightly soak a paper towel with vegetable oil and wipe the whole pan down so that it is lightly coated in oil. This will prep it to be cured in the oven (so the oil doesn't spoil)

To cure it, I usually don't set the oven too hot. 350 degrees Fahrenheit or so for 30 minutes is usually good enough for me. The point is to get it hot enough to cook the oil but not to burn it. Afterwards, I will wipe it down lightly with a dry paper towel if there is any excessive amount of oil and just let it cool in the oven with the heat turned off. The pan should be somewhat shiny in the end.

Hope that helps.