If stripping down to bare metal, old school oven cleaner is lye. Just put a pan in a garbage bag, spray the heck out of it, let soak a day or two, rinse with hose and repeat if needed.
If they are stripped to bare metal, going over them with a random orbit sander to take off the high points is well worth the effort before seasoning.
Restaurants quickly season pans by baking them at 450 to 500 degrees for 30-60 minutes. Many of the lower temp traditional ways seemed to make sense for a lot of years, but I see people all the time with mushy seasoning and it’s all I can do to not suggest they ignore what grandma taught them years ago and just give em a good bake.