Yeah, sandblasting will take off EVERYTHING, instantly.
Check your local listings for a sandblasting company. I've had them do a whole 55 gallon drum, inside and out, lid and all for only $25.
Just be aware that you'll end up with raw, bare metal and it will start to rust very quickly when exposed to air or moisture. Just spray the grates down with veg oil when you get them home, then heat to season.
Big JT's Smokin' BBQ Competition Team
"There is a freedom that cannot be taken from you. There is an inner freedom that nobody can ever deny you of, even if they lock you in a cell and block you off from the world. Seek that freedom."