if minor, a brillo pad.. or depending on your pan size, use a few teaspons of kosher salt with equal parts of vegetable oil .. with a folded paper towel or rag, vigorously rub the entire pan - of course concentrating on the rust spots. whatever does not come off that way, use a steel wool to remove the remaining.
Here is a past article on how to clean/re-season it too! http://www.hometalk.com/120827/help-how-do-i-get-rust-off-an-old-cast-iron-cornbread-pan
Don't forget to season the cast iron after you have cleaned it.
If it has been used or seasoned before use a sos pad and rince and dry and rub oil into it. Always make sure after using it and washing it dry it and rub oil into it before you put it away. Thats what I do with mine.
Depends what you mean by "really rusted"
If pitted don't bother.
But if this is just a layer of dry surface oxide, a swish an soak with white vinegar and steel wool will get you started.
We use CI all the time and have over a dozen good spiders. To season it the first time, we use the oven to bring it up over 200° for a good 20min with oil in it.
Heating the cast iron opens up pores to let oil work in and make it into a non-stick surface
Clean after using and rinse, then wipe right away. Don't put them in the dishwasher and don't let them sit with water in to cause rusting again. I wash with the sponge and dish soap, then set it on the range top burner to warm it dry and add oil and wipe with paper towel right away. Then it is all ready for next time
steel wool, yes. soap, no.
I've reclaimed lots of rusted iron skillets and pots by using a rotary wire brush on a drill to clean the bad rust off. Then scrub it good with a Brillo pad. Once clean and rinsed, dry good quickly, like by placing on a hot stove. Rub down with unsalted fat of some sort or oil and I've even placed them in the fire of a bonfire or woodstove and then repeated the oiling after they cool. Don't leave them in a really hot fire or all your oiling will burn off and will come to naught. You just want to season it. After that you can use it and wash it lightly, re-oiling as needed. Just make sure you dry it and don't use harsh detergents on it that will remove the oil.
I want to thank everyone for their suggestions. I am getting on this on the week end.
if pitted, can have it sand blasted, I had this done to a huge fry pan, so big I could hardly lift it myself, and it came back smooth and shiny.
Use baking soda to scrub them, then wipe olive oil all over them. Wipe the outsides off after treating with oo and never put them in water. Allow the inside to sit and soak up the oil.
Since I use these cast iron skillets all the time, I give them a good scrubbing with SOS pads and wash then dry immediately, I usually dry mine on a low heat stove burner to dry. Then coat the skillet with a generous coating of oil and place in a 350 degree oven for a couple of hours, this will season it.
When the cast iron begins to look dry just clean and season it again. I clean and season my skillets and dutch ovens about once a year.
Look what I came across in my browsing
I never use soap on my skillets, but I still cannot give up rinsing them in water! But lately i have begun to think I better let go of that idea too! I use a razor blade to scrape anything stiking to them. My mother gave me one that was so old it felt like satin on the inside! Sweet!!!!!
once you get the rust off, steel wool and cooking oil cuts it off , you can wash them when needed with soap, must be old fashioned soap not detergent! My skillets are over 100 yrs old and they are slick and no stick, eggs slide in them. cook gravy in them and just clean with water only, then dry, they love it!wipe with oiled paper towel rub in good before putting away, i hang mine on the wall.
Build a fire outside (or in a wood burning fire place) when the wood has burned down to glowing coals put the cast iron pan or pot in it, let it burn down and cool on it's on. After it's cool scrub with soap and water, then season with shorting. I was taught this by my Grand Mother and have been doing this for 45 years myself. I love to get cheap yard sale cast iron pans and reuse....