Paul I don't know about using Flax Seed oil. I know that on my older char broil grill I had older heavy duty cast iron grates that I would wipe down with peanut oil after every grilling. I had those grates for years and never had a rusting issue.
Yeah, Muebe, It must be me. I only know of nine people who are complaining about CI grate rust and Cookshack has probably sold hundreds of these things. I had the same problem with the stainless steel grates on my old Char-Broil Quantum while other people didn't seem to.
All I was saying is that I am not aware that Flax seed oil works as well as peanut oil for seasoning cast iron. And also note that I would "wipe" down the cast iron grates top and bottom. If I did not do this they would have rusted for sure! I used to run all four burners to get everything burned off after cooking. I would peg the hood thermometer at 800F. Then after cooling I would need to re-season the grates.
The direct flame of the FE is going to burn off the seasoning. So constant re-seasoning each time would be required IMHO to keep them from rusting.
And relative humidity of where you live will have an effect. Also living near the ocean for example due to the salty sea air that will eat away at any exposed metals.
