Cast iron barbecue grates, just like cast iron skillets, need to be seasoned and maintained. I found a number of websites that offered tips on seasoning barbecue grates. After doing the research, this is what I did:
Prior to first use, wash the grates thoroughly with a mild dishwashing liquid to remove the protective wax coating usually applied by most manufacturers to protect the iron during shipping. Rinse the grates in hot water and dry completely with a soft lint-free cloth or towel.
Pre-heat your oven to 300 degrees. Spread a thin coating of vegetable shortening over the entire surface of the grate (top, bottom, corners, sides) with a clean cotton rag that you don't mind throwing away when your done. Some people said olive oil or other oils would work but the Weber site specifically recommends:
A solid vegetable shortening is recommended for the initial seasoning. Spread a thin coating of solid vegetable shortening over the entire surface, including all corners, with a paper towel. Do not use salted fats such as butter or margarineI started with paper towels as recommended but I quickly found that they began to shred and leave bits of paper behind. So I used part of an old cotton t-shirt that was in the rag pile. I also did my application outside and put about 4 sheets of newspaper down which was a good move.
Cover the oven shelves with foil and place the greased grates in the oven for 2 hours. After, turn off the oven and let the grates remain in the oven until cool. Repeat after grates are cool.
Once they cool for the second time, they are ready for use.
To maintain them, you should re-season them regularly.
The Weber web site says:
To prevent rusting, they should be re-seasoned frequently, particularly when new. If rust occurs, clean with a steel brush. Reapply vegetable shortening and heat as indicated above.Before putting the barbecue away for the winter, grease the grates very lightly with vegetable shortening, then wipe dry with a lint-free towel, wrap tightly in plastic wrap, and store them in a dry place.
The Weber website does validate one of my practices:
Don't do a burn-off after you grill, but rather leave the cooking residues on the grates/griddle to keep a protective coating on the cast iron. Then do a burn-off just before you grill. Brush off charred residues with a steel brush rather than a brass brush.Those "cooking residues" on most barbecues are going to consist of animal fat which is a great product for the cast iron. But, in this household, there's going to little to no meat cooked on the barbecue. So, perhaps, more frequent seasoning will be required.
Here's how it looked after seasoning.