Cast iron pan clean or oil?

You can clean off the rust using half a raw russet potato. (The Kitchn and Apartment Therapy have posts on this.)

If you can get it cleaned up, it's worth every bit of effort. I bought my 10 inch at an auction of household goods 40 years ago. It was seasoned already and just needed some cleaning up. I have never had to season it again. I avoid soap and water unless necessary. It is the most perfect nonstick pan you can imagine. Over the years I have picked up two more smaller ones. Same story: clean up but no seasoning necessary!

This very site will tell you how to season your cast iron pan: https://food52.com/blog/3547-how-to-season-cast-iron Just scrub all that rust and anything else off the pan first and dry well.

I generally follow the Lodge company's instructions for seasoning cast iron cookware. http://www.lodgemfg.com/useandcare/seasoned-cast-iron

I don't use soap on mine, I just make sure to rinse the pan straight away after cooking and put it on the cooling stove so it dries quickly. As trampled by geese said though, there are as many 'correct' ways out there as there are people!

For an old cast iron pan, what I do is season it. There are almost as many ways to do this as there are cooks on this planet, most of them claim to be the only or right way to do it - but in reality, most of them work well. I've tried a few styles and here's what I find works for me.

First wash the pan with soap and water really well. Get some super-fine sandpaper and smooth out the inside if necessary. Only if necessary. Wash again.

Second, smother (really smother it, there should be puddles) it in an edible oil with a high-ish smoke point - like grapeseed.

Put it in the oven and cook at 350 F for 1 hour. Leave it in the oven as it cools. Next we wipe out the excess oil that hasn't absorbed yet. If you can catch it at the stage where it's cool enough to touch but not so cool that the oil solidifies, that's best time to wipe out the oil.

Now it's ready to use and shouldn't rust, depending on how you use and wash it.

I usually season my pots twice a year (on the equinox) if my pots need it or not, but more often if they start rusting in between use.

Admittedly I'm one of those evil people that wash the cast iron with soap and water (my own pots, never someone elses). Most people insist that they are washed with salt and a damp cloth, and this works well enough but not for how much I use my pots. The key to getting away with using soap and water is to have a well seasoned pan, never soak it, and make it so it will dry quickly. I use my cast iron skillet at least twice a day, and usually one or other cast iron pot as well - When I wash with just salt then too much flavour transfers from one meal to the next. Soap works just fine for me.

Anyway, there are loads of opinions on the internet. Try searching "how to season cast iron pot" and "how to wash cast iron pot" for an overwhelming plethora of methods.