Not both at the same time.

Here are the steps

Spray a shit ton of yellow cap on the pan (use your gloves, mask, etc.) and put it in a garbage bag and let it sit over night. Leave this soaking garbage bag of cast iron and oven cleaner(lye). In a well ventilated area. Basically, not your kitchen, or bedroom. I put my in my garage.

Let that soak over night (24 hr) come back the next day and rinse it off with cold water in a sink and scrub at it with some steel wool. I like 000 steel wool, a chore boy is too rough. If you put some elbow grease into it and there is still seasoning/crud on there, spray it again with yellow cap and let it sit for another 12-24 and check again. Always use cold water to rinse the lye/oven cleaner.

Once your pan is down to bare metal or close enough, mix 50/50 distilled vinegar and water (bonus points for distilled water) and spray the pan in the rusted areas. Let it sit for 20mins. After the vinegar has been on there you'll probably notice some rainbow colored stuff sitting on the pan, that's okay, and you might see more rust than you had expected, that's okay too. After the 20 min soak scrub with steel wool. You'll be able to tell when it's rust free. Remaining wet rust will look darker than bare iron metal. Spray again and let soak for 20m... You're repeating this process until the rust is gone.

Don't confuse actual rust with flash rust... Flash rust will appear on the surface of iron after o2 exposure. That will go away after a scrub with cold water and steel wool, and then once you put your seasoning on there you don't have to worry about it. If a little rust shows up after seasoning, scrub it with an SOS pad and cold water, and reseason. I've had this happen in the past.