I have done this before. Put the oven racks in a big plastic trash bag pour a
bottle of ammonia in the bag.Close bag tightly with a baggie tie and soak
overnight.I do this outside because of the possibility of fumes. The next
morning, outside, open trash bag away from you because of the fumes. Then just
spray with the garden hose and the grit and grime sprays right off. I did this
with a stove we bought. The stove was clean but the oven racks were awful.After
soaking in ammonia they looked like new again.