You didn't say how big the grates are. One method I've used is to stack the grates in some sort of plastic tub (old toddler's wading pool?, cattle trough?) add a cup or so of ammonia, then run a hose from the drain of your hot water tank and fill the tank with hot water. Let it soak a good long time then take the racks out and pressure wash them. They still won't be shiny but there's no point in removing the seasoning.
You can do a lye soak as well, bit more dangerous.
A word of caution on sandblasting. Where I live sandblasting rates are upwards of $110/hour and they'd probably charge you for an hour. Also, if you get some knuckle-dragger who's set up to run coarse blasting sand used on ship hulls for example, those can literally blast right through expanded metal. We used to see that in the shipyards where we'd weld circles of expanded metal over bow thruster tubes and when the hulls got blasted prior to final painting sometimes the sandblasters would destroy the expando. So I'd be really selective about sandblasters both for price and for quality.