Have to agree with JHayden. Spraying with some oil like a wd 40 or engine based spray oil and wiping with rags is best option. Additionally, I've used the DIY sprayer at the carwash. However, only using the mist setting. And only when it was absolutely necessary to get off some road rash or something off the bottom. since my engine compartment isn't completely sealed off from the elements I've found that this misting helps with getting general dust off and some residue. Finishing by drying everything off in the bright sunshine repeatedly until there is NO visible moisture.
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