Seasoning, or curing, a grill grate refers to the method of building a layer of cooked-on grease onto the grate to prevent food from sticking to the metal. This is most often a concern with cast iron grates because cast iron is a porous material containing many small imperfections that food can sink into and stick. Seasoning the grate fills in those imperfections and gives you a smooth, non-stick surface for grilling.
Wash the grate with liquid dish washing soap and hot water. Rinse it thoroughly and dry the grate completely with a clean dish towel.
Spread approximately one tsp. of solid vegetable shortening on a folded paper towel. Wipe the shortening all over the grate, making sure to cover the entire surface with a thin layer of shortening. Use more shortening if the size of your grate requires it.
Preheat gas grill to medium heat for 15 minutes, and place the grate in the grill. Close the lid and allow the grate to "cook" for 60 to 90 minutes. Turn off the grill and let the grate cool. If you have a charcoal grill, arrange coals according to the indirect heating method and light them. Put the grate in place, close the lid and heat the grates for 60 to 90 minutes. Close the grill vents and allow the grate to cool as the coals die out.
Reseason the grate any time food sticks to it. Do not wash the grate with soap when reseasoning. Expect to repeat the process several times before your grate is perfectly seasoned.