You can buy a cleaner especially for them. There are several brands. Here's one to check out: https://amzn.to/2VBUg1H (affiliate link)
Cover the pans in salt and then pour some white vinegar on them. Add more salt and let it sit for 15-20 minutes. Scrub with a sponge with some salt on it. You can also use a lemon in place of the vinegar.
Hi Tom I hope this video helps
Here's what one Hometalker uses
I use vinegar (white is stronger for cleaning than the yellow apple cider kind) and regular table salt. Sprinkle the salt over the pan after dousing vinegar on the pan. Use a scrubby sponge to scrub. Easy, cheap, and works great.
Check the acidity of the vinegar. Typically generic white vinegar and apple cider vinegar are 5% acidity, "cleaning vinegar" is 6% acidity and goes up from there for janitorial and agricultural uses.