Having cleaned a number of DO's for my son's Scout troop, if there is no rust and just crud in the DO, then just fill it with water and boil it for a while then empty and scrape it clean, wipe it down with oil while still hot. If it has significant rust, then clean it it with a wire brush, ( I have sometimes with a brush on a power drill) and then wash the DO and then re-cure it.
Lodge has some good info on this
http://www.lodgemfg.com/use-care-seasoned-cast-iron.asp