If you are using your Blackstone for the first time, follow Blackstone's directions, they work=> take some soapy water, clean, and dry the surface of the griddle. After it's dry, turn on the griddle on high until it starts to smoke. Start with about 2 tablespoons of the Blackstone seasoning, spreading it around the griddle (use more if needed). Leave the griddle on high until the grease burns off, turning the griddle surface black. Repeat.
If you have spots towards the front where the griddle haven't turned, pick up the griddle (using welding gloves or similar), detaching the feet from the flame box, and turn so that the front faces the back. Repeat the grease process and you should have a blackened/seasoned surface. Replace the griddle to it's original position.
Lastly, spread some vegetable oil of your choice around the griddle top, let heat until it starts to smoke and turn off the burners.
From here it's up to you to make sure you keep your griddle top clean and oiled after each use. I've have a 22" model that I've used for 3 years and have never had any rust on it.
Enjoy!
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