It’s often helpful to sand & polish cast iron skillets before seasoning them. You just have to take precautions so you don’t inhale the metal shavings. Here’s a good reference: https://youtu.be/2wHZ7hKpPUs

Also, the type of oil you use to season your cast iron cookware is important. You should use flaxseed, grapeseed or avocado oil. Do not use olive oil, because it has an extremely low smoke point. Canola and vegetable/soybean oils are unhealthy.