In my experience you need a strong solvent to get the dirt out and something just a little corrosive to make it nice and shiny. Soak in a 1:1 mixture of battery acid and petrol for 20 mins. Then neutralize it with some thing alkaline like Mr Muscle sink un-blocker. The a quick rinse and your good to go.
P.S I’m sorry please don’t ever do any of the above.