Quite a few years ago I had a friend that worked for a large trucking company. While sometimes working as the scalemaster on the yard he would work late evenings. When working the late shift, and being pretty much the only one in the yard, he would let us bring our cars in and use the truck wash bay. The bay had all the cleaning tools you could think of, including a steam cleaner. The bay was designed so that you could get to the underside of the vehicles rather easily and give the vehicle a good cleaning top to bottom.

While I'm not suggesting sneaking into a trucking yard to give your a truck an underside washing, perhaps you could find somewhere where they would let you use their bay for a one time cleaning?

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the sharpest tool in the shed
1989 Mustang 5.0 GT (25th Anniversary Edition)
~hobby car~