If you cant find a Listing for Sand Blasting, you might call an Auto Paint facility.... they frequently have sand blasting equipment.

Don't know how I missed this. I had a big cauldron that my honey had sandblasted and as Dale said we had it done at a paint shop. A friend owns it but did a great job. After the house burned down someone stole it before we could get over there to start salvaging what was left. I had to look at it as Goddess thought someone else needed it more than I[confused]

I just skimmed through but I don't think I saw this posted. A few years ago I was watching a television program that suggested the following for cast iron skillets.

Make a paste of salt and water. -Not a great amount of water just enough so the salt clings together.

Place the pan over medium heat and allow to sit for a bit. You should start to see some of the rust color come through to the top.

When you see this turn the heat off and allow the pan to sit for a minute to cool.

When completely cooled take a washcloth and scrub the pan.

You can repeat this as many times as you want.

This was an alternative to using chemicals such as comet or those soapy brillo pads.

I have had to do it several times cause when I perform a Druid rite sometimes I forget to empty the Well. Silly me. [oops]

I can not believe I forgot about using salt. I've worked in resturants all my life and salt is used to clean cast iron and other things. Thanks Tommy. [smile]

I know this works for chrome, but I'm not sure about cast iron;

Crumple up some aluminum foil, and dip it in Coca Cola. Yes, you read that correctly. [cool] Then rub vigorously. It takes the rust right off of chrome car bumpers, like the set on my mother's '75 Ford Mercury Comet (don't try it on plastic though!), but like I said, I've never tried it on cast iron.

Then I'd say get a small bottle of black Rustoleum, but Fate does have a good point about heat and flame, so check the label for flammability before you buy. If you decide not to retouch it with paint or other substances, just remeber that the best way to prevent further oxidation is to clean up promptly after ritual, and dry your tools thoroughly.

Also, if you try the suggestions of everyone here and still need more help, try the House Elves forum, it's devoted to cleaning advice.

Keep us updated.

Scotchbrite pads from 3M industries should work well for removing the rust from small cauldron, sandblasting should be reserved for the larger cauldron. Also since I assume you will be using these cauldrons for rituals, avoid the use of harsh chemicals, such as CLR, acid treatments, etc. Once the rust is cleaned off then "season" the cast iron using olive or clear vegatable oil,one of the other posts have good description of the process - but please don't use motor or mineral oil (Great-Grandma never did forgive me for that lol).
Since you will use these items for ritual purposes spending time and effort cleaning and seasoning doing as much by yourself will infuse your own energy into the cauldrons much in the same way you would your wand.

Like Artemis said....there should be a rustoleum that is flame resistant used for BBQ Grills! [cool]