Jet wash all mud from under the lip of the rear arches, most dc5's will have this here! Ensure area is clean and apply a good underseal with a brush (spray underseal actually traps air pockets which can still lead to corrosion underneath), everytime you wash your car ensire arches are washed etc, apply a good wax such as collinite 476 to the rear arch area to drive water away, sorted!
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