How did rust appear on my cast iron in a matter of minutes? So this has been an on-going saga the last few days, and I've already posted several related questions here, and I am feeling quite defeated.

I have/had a beautiful 16" cast iron dutch oven from Cabelas. I took it out for a test spin the other night (I had completed all seasoning steps that the Google Gods recommended). Then:


I tried (and failed spectacularly) to cook a pork roast in it, but cooked it at way too hot a temperature and burned/charred/fused the animal fats/juices into the oven, whereby it basically baked into the cast iron (related question here)
I tried cleaning/scrubbing it with coarse-grained salt -> did nothing
Then I tried cleaning it with dishwashing liquid + salt -> did nothing
Then I let the whole thing soak in a vat of a 50/50 mixture of water + white vinegar -> took all the tarry slude off, stripped some seasoning off (revealing bare iron), but in 80% of the burned/charred areas, still did nothing
Then I went to town on it with a steel wool and salt and dishwashing liquid and was left with some smooth, properly seasoned surfaces, and some more areas stripped down to bare metal
I decided to just re-season it. So I scrubbed it one last time (about 1 hour ago) with a steel wool and some water, got all the rust and remaining crud off (finally), and built a fire.
I placed it on the hot coals of the fire and let it sit there for 15 mins. My intention (whether misled or not) was to burn off all the remaining water from when I had previously washed/scrubbed it, then let it cool off, then season it with oil, and then bake it for a few hours.


So I just took the oven off the fire and opened the lid...and the entire inside is covered in rust. An hour ago there was none. When I put the oven into the fire 15 mins prior, there had been none (yes, I checked) This just came out of nowhere.

I'm ready to give up, unless anyone can offer some concrete set of steps for me that will salvage this $100 rust bucket. How can I get rid of the rust and then re-season this before more rust sets in? Meh...
The problem with your last step was the lid, I think.  

If you had placed it on the hot coals, open, the heat should have driven the water off as you intended, drying the pot before it rusted.  With the lid on, the moisture was trapped inside, and had opportunity to cover every inch of the metal - and, as Joe mentioned, heat speeds up the reaction.  Sitting on the heat with a coating of water, and you have a layer of rust.  Not functionally different that if you'd tried boiling a pot of water before seasoning it.  The seasoning is, after all, supposed to protect the bare metal from water so it doesn't react like this when being cooked in.

How to fix it - go back two steps.  Scrub off the rust, or as much as will come off easily.  I agree with paparazzi, here, if it won't come off while scrubbing it will probably not come off while cooking - especially under the seasoning.  Once it's well scrubbed, dry it off.  You might use a towel or something first, but if you're going to put it on the fire, leave the lid off so the moisture can escape the pot.  

You might have the lid drying to the side, leaning against the outside of the pot, or partially on (generously offset) if you want to dry the lid as well.  You probably don't need to heat it for long, either, just until it looks dry - and it will be evaporating water all the way until it cools down, too.  When I'm re-seasoning my frying pan, I heat it only 10min or so, and by the time it cools it is usually dry (disclaimer, I use the stove rather than coals, so your timing may vary).

After that, you can smear it with a layer of oil and re-season.  wipe it down with oil, and heat to re-create the layer of seasoning that got scrubbed away in the first place.

Beyond that, it will probably help to calm down a bit.  In the future, just remember to keep the lid off when drying, and that you don't need to panic quite as much.  Cast iron is pretty sturdy, and scrubbing down to the metal and re-seasoning is always an option until the pot is worn away to holes.