if it was me, I would find out who has a sand blasting tub and clean it out with sand. Then wash it out and heat it dry then reseason it. Sand blasting will get all the rust off. shouldn't take too long to blast it off. If you paid some one to do it, it would be cheaper to buy a new one.
Wash with soap and hot water, scour off rust (SOS pad or wire brush), and reseason:
1. Apply a thin coat of melted vegetable shortening (i.e. Crisco) to the entire surface (including lid if applicable), both inside and out.
2. Line the lower oven rack with aluminum foil and preheat oven to 350° F.
3. Place cookware upside down on the upper oven rack, and bake for one hour.
4. Turn oven off and let cookware cool before removing from oven.
5. Store in a cool, dry place. If you have a lid for your utensil, place a folded paper towel between the lid and the utensil to allow air to circulate.
After use: Clean using a stiff brush and hot water only (do not wash in dishwasher). Towel dry immediately and apply a light coating of vegetable oil to cookware while still warm.
Been doing this to some scouting dutch ovens that got overlooked following a camp-out!!
2005 Jay Feather 29Y
2004 Nissan Armada LE 4x4
5 Happy Campers
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If the pan is too difficult to clean with S.O.S or has a grease build up you can put it in a self cleaning oven and run it through a cycle. Thing is you then HAVE TO completely re-season it.
The best method that I found to remove rust from iron or steel objects was to soak in a solution of Oxalic acid and water....about 1/2 cup to the gallon of warm water. But depends on how bad the rust has penetrated. If real severe, first wire brush the crusty parts off before dipping. Let soak for a couple of hours then check to see how fast the rust is disappearing. A whitish/green coating will appear on the pan when the rust is being dissolved. When the solution starts to turn dirty green, then you know the acid is working. It will dissolve rust down in the pitted areas which a scrub brush cannot reach. When you are satisfied all the rust is gone, then rinse thoroughly with water, let soak in a baking soda solution for a while, then rinse with water again. When done, then regrease it with lard and bake in oven to ge the oils back in the metal. From then on, do not scrub with soap, but rather scrape off food and wipe from pan and regrease lightly before storing away.
I used this acid to clean up old rusty beer cans that my son was collecting and it works great. It didn't ruin the painted labels on the cans, either. Also, works great on cleaning up a cars radiator system, bolts and nuts, car parts, etc. Oh yeah, use rubber gloves too and plastic containers!
Your druggist should have some Oxalic acid in granular form in stock....but if not....he can order it for you. Or try a auto parts store. Good luck.
Bob and Barb
2005 Rockwood 2304S
1999 Chevy Van Conversion Van
Equalizer WD hitch
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Similar to Reddytoroll I have used white vinegar full strength. Just pour enough vinegar in pot to cover the rusted area and let it set for a couple of hours and check to see how it's doing. Leave in longer if necessary. Then rinse good, dry and reseason.
I agree with Dinosaurman, use a food product and mild acid. VINEGAR!! Then reseason. I use peanut oil to season as it has a high smoke temperature. marly
I was given an old frying pan that was found under a house. I used a wire brush and then an electric palm sander with 80 grit, 120 grit, and 400 grit sandpaper. I scrubbed and rinsed it after that. Then I reseasoned it in the smoker. I seasoned cast iron in the oven one time. It stunk up the house and the DW was not very happy.
2000 GMC Sierra Ext Cab
4.8 L, 4.10, bilstein HD shocks, roadmaster active suspension, Hypertech computer tuning.
2001 Fleetwood Prowler 27X 5500 lbs loaded.
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left a cast iron pot where she shouldn't have and the interior bottom is all rusted... YUCK!!! 
and two teenage sons.
