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You can use any low sudsing laundry detergent or the chemical suppliers of shop chemicals will have floor detergent that can be used. I would start with half the recommendation and move up if the parts are not coming clean but they should as long as the heater is working.
All ferrous parts will begin to rust as soon as you remove them from the washer, it depends on the application if you want to coat them with WD-40 to drive the water off them or if a light petroleum oil is better suited. The aluminum will oxidize as soon as it is exposed to the atmosphere so a plastic spray clearcoat on the external portions will keep that clean look. The inside will have a spray oil.
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