Normal new A36 steel plate generally has mill scale, dirt, rust on it to one degree or another. I sand blasted mine down to a "white metal" finish which removed all scale, rust and grunge. I seasoned the plate by oiling it with vegetable oil and baking in a 550F oven.
You can look in the yellow pages for local sand blasting companies . Make sure you specify a "near white" or white metal " finish (not standard industrial finish) as the price depends on the finish.