use a solution of muriatic acid and water. just like they tell you with most detergents and delicate stuff, adjust the solution and try each time on a small section. you could start with say a 50/50 solution and adjust as needed. needless to say caution is key when using this stuff. rubber dishwash gloves and a respirator because you'll be using a stronger than normal mixture. cover your fenders with plastic visqueen or some heavy duty garbage bags for large trash cans. and try to avoid doing it in a windy place. have a jug of water nearby to rinse the plenum with after it gets to the desired cleansed point you want.
