Clean the pan really good with steel wool or SOS pads. Get rid of the rust, then use lard to season it in your oven, rinse and season again and again until it's perfect.

I cleaned up some really oldies last summer or the year before...collected from yard sales...then stuck up in the attic of the barn for years. DH went at them with a grinder wheel on a drill, oiled, baked on the grill, re-peat. A bit of work but worth it. Lots of methods depending on how rusty they are.

(an oven is the best way to heat, but a couple of them are huge) way bigger than our oven.

The bear cornbread pan we 'fixed' last thanksgiving took some work. Now, after a few batters, it is close to non-stick...it needs another scrub me thinks. I use old saved corn flour to bake and test them. Helps absorb the funk. Toss it out. Usually baked at the same time as our other ol' faithful cast iron...Best to scrub and get it right the first time but DH ran out of steam....

Take a look at this thread from 2012. THIS THREAD.

Using 'easy off' or ammonia in the hot sun is easier than the grinding approach and even more an advantage with odd shapes.

I did try that method, with ammonia. On some sheet pans and a rusty cast iron. I kept checking for days but it did work...way too impatient when we were anxious....job done, check.

It had some trouble for me with super crusty rust. And looking at that bag day- in-day out, got a bit creepy...checking, forgetting, checking, stink!...an ugly mess and still needing cleaning. Horrid fumes.

The ammonia is slower (less aggressive). You also need hot sun. The easy off is faster. I usually just wait one day (impatient just like sleevendog) and things are done without anything more than a fiber sponge and perhaps a little emery cloth or 'wet and dry' sand paper on a few leftover spots. The wet and dry thing is also good for regular burned spots on any pan.

I have stripped 4 vintage Griswold CI skillets this last year as gifts. After much research, I went with the method of Sheryl Canter. You can do a search of "Chemistry of cast iron seasoning Sheryl Canter" and pull up the info. I used Easy Off oven cleaner to clean these skillets as they had many layers of baked on crud. I did it wearing rubber/latex gloves, in the yard on layers of newspaper and let sit for 30minutes to an hour and then used one of those razor blade paint scrapers to remove the gunk. It took numerous sprays to get it down to bare metal. I then used fine steel wool (0000, I think) to get it smooth like glass, then did the six coats of baking/seasoning with the flaxseed oil. They are now real beauties and the finished product (seasoning) made me a believer. I wasn't in a rush, so did this over a period of days. Your pan sounds much cleaner and being a muffin pan (rather than a skillet) it probably doesn't have the build up, so you may just be dealing with rust removal. I have seen those muffin pans on auctions and been tempted, so would love to hear how you like it for baking.

Now your big job is to bake some muffins and tease us with a photo of them in your beautiful seasoned pan!