I just purchased a 12" cast iron skillet from Lodge. Even though it came pre-seasoned, I've read that I should season it a few times in the oven so I can get a good, base seasoning to the surface. Well, I used vegetable oil, spread it around, baked it in the oven for an hour, and I thought it would come out perfect. Well, after taking it out, I noticed there were two round spots on it. Inside the spots the surface of the pan is rough--the rest of the pan is very slick. Did I do something wrong? Did I use too much oil? Not the right type of oil? Thanks!

I've included the image here: