The most basic form of body maintenance is to wash it regularly with plenty of water (using a hose). This helps remove any solid particles that may have stuck to the car. It is important to remove all these particles during the washing process to prevent future scratches. It is essential to wash the wheel arches and subframe to remove all the dirt that holds moisture and accelerates the rusting process. It may seem strange, but the best time to clean the subframe and wheel arches is wet weather, when the dirt remains damp and soft. In very wet weather, the subframe usually automatically clears itself of large build-ups of dirt and this is the best time to inspect it.
Except for vehicles with a wax-based underbody coating, it is recommended that the subframe and engine bay be steam cleaned periodically to allow a thorough inspection to determine if minor repairs or replacements are needed. Steam cleaning is available at many garages and is essential for removing oily dirt. If steam cleaning is not available, cleaning with oil solvents applied with a brush may be possible. In this case, the dirt may simply dissolve.
It should be noted that these methods should not be used on cars with a wax protective coating on the body, otherwise the coating will be removed. Such cars should be inspected annually, preferably before winter, with the underbody washed and damaged areas of the protective coating restored. A complete replacement of the coating is ideal. It is also advisable to use spray wax protection on door panels, window sills, etc., as this provides additional protection against rust in places where such protection is not provided by the manufacturer.
Always ensure that the drainage and ventilation holes and tubes are fully open so that water can flow freely through them. A dirty film may occasionally appear on the windscreen and windows, which should be removed with a suitable glass cleaner. Never use wax or polishing pastes for bodywork or chrome parts to clean the glass.
[Publication reprinted from this resource: MercedesMan]