I've only bought the pan a week ago and used it three times. Hot dogs, eggs, and steak.
When I'm done I clean it with a scrubby sponge and once with a little bit of detergent, then put it on the stove to dry quickly. Then while it's hot I smear maybe a teaspoon of vegetable oil on it with a paper towel "brush".
In between uses it's wet with oil, as you can see in the picture. How much residual oil should there be? I had the impression that it would be dryer.
Also, how much should I scrub? I am not going to leave crust of beef on there, but I also don't think it's supposed to be scrubbed back to new smoothness.
How does that work? I’d imagine it would be the opposite. Less time with any residual water particles means less chance it will sit there and react with the iron. Or at least be the same.
Good question! Rust, or iron oxide, is made from oxygen forming an ionic bond with the iron. Water is great at getting into the pits and cracks and crevasses of things like cast iron and will sometimes release oxygen to bond with the iron.
Creating iron oxide requires a decent amount of heat an energy, although less than the reaction puts out, allowing for runaway heating. Adding heat to your cast iron and the water can make the iron-oxygen ionic bond easier to facilitate. On top of that, adding energy to water makes it easier for it to break down into hydrogen and oxygen, leaving the oxygen available to create rust. The runaway heating from the rust reaction can cause small bits of water that you can't see continue rusting even after you remove the skillet from the heat of your stove.
Since the whole process of drying your skillet using heat works by increasing evaporation, you're freeing up more oxygen to bond with the iron, heating up the iron to more easily facilitate the ionic bonds, and expanding the metal to allow more water into areas that'll continue holding the water after the skillet has cooled.
Oddly enough, if you needed to clear rust from cast iron, you can more easily melt or break ionic bonds using even more heat, like a flamethrower or torch.
Does this take into consideration how much longer the water will be there as it evaporates at room temperature to how much shorter there will be any water present for any reaction when boiled away instead?
I guess it depends on what you mean. Either way, you should try your best to dry off your cast-iron utensils with a cloth or something like that. If you're rinsing your pan, pouring off the water, and then just setting it down, you're asking for rust either way. But, applying heat to evaporate the water is giving the oxygen atoms a better opportunity to form ionic bonds. And remember, not all evaporation is due to heat. Heck, it would be interesting to find out if boiling off your water by quickly pulling a vacuum around the cast iron would be an effective way to reduce rusting.
If you like to leave your cast iron pots and pans very wet, then you're probably creating a pretty good environment in which the water can absorb carbon dioxide and form carbonic acid (the thing that makes a glass of water that's been sitting out all day taste "stale"), which will really exacerbate the rusting.