IMO your undercarriage goes through hell during rain- rocks, twigs, muddy water in the cracks and crevices, etc. That stuff is acidic and it can corrode your car. Get the undercarriage washed but dont let them touch the paint. Theyll screw it up ( guaranteed). Also dont let them spray anything but water. No oil, kerosine, etc.. you can take some meguiars or 3m degreaser with you if it is excessively dirty.
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