Study by The Technical University of Munich, Germany
The Technical University of Munich has carried out an impartial scientific comparison test in association with Mercedes-Benz to establish the exact ranking of hand washing versus machine washing of vehicles relative to damage to paintwork. To make the test as realistic as possible, typical family sedans were used - with a few hundred miles on the odometer. It was stipulated that each car was to receive the equivalent of one year's wash stress (about 25 washes) - some cars by hand washing methods, others by automatic car washing equipment.
To make the test as realistic as possible, and to give both the hand washers and the automatic car washing equipment something to get clean, five test vehicles were thoroughly coated with a mixture of street dirt, under-fender dirt accumulations, oily water, and thawing-salt residue. This test mixture, inspected under a microscope, contained grit particles measuring between 0.5mm and 1.0mm which are enough to damage the toughest paintwork if not carefully removed.

The Hand Wash
Without having been told the why's or the wherefore's, four men were given the chore of washing four of the test cars by hand. Each person was to do as thorough a job as possible. Since there were four people involved, and since each person used their favorite "tools", together they presented a cross-section of various hand washing techniques commonly used by average car owners.

So the marathon carwash began. The four men showed up in four different sections of town enthusiastically sponging and lathering their test cars as if they were their very own. Bystanders could see one man hard at work in a do-it-yourself self-service type of car wash with high-pressure water facilities; another was busy at home with the garden hose and a water spray-brush. One man scrubbed away utilizing a bucket, a sponge, and a garden hose. One, to the horror of the environmentalists, and in defiance of local regulations, washed his test car at a lakeside.

The Automatic Wash
As the alternative to the perspiring hand washers, automatic car washing equipment was used to wash the fifth test car. To keep the test above board, the test vehicle arrived unannounced during the hours when the car wash was open for business. The car was washed along with other cars.


The Examination
To measure the extent of paintwork damage, microscopes were available. However, pure reflecting microscopy, as well as electron microscopy results merely in a visual picture of scratches. A comparable measure of true depth cannot thus be achieved. Consequently, a method of analysis was used which could determine the exact depth of the scratches and grooves, namely, the interference-microscopic analysis method.

This method allows the glossy surface, with all of its irregularities to appear like a relief map. An interference band is created and corresponds to the contours. Band distances are 0.27 of one thousandth of a millimeter, thereby allowing even the tiniest grooves and scratches on the gloss to be accurately measured and compared.

The Result
The comparison tests produced results of which every car owner should be aware. Automatic car washing and hand washing have completely different effects on automobile paint.

How does the paintwork of a car appear after 25 washes - equivalent of a year's worth of washing?

The results are depressing, at least to the car owner who, up to now, has firmly believed in hand washing. The deepest scratches after hand washing had a depth of more than one-tenth of the paint surface. Under the microscope, the paintwork looked like a cratered landscape. The paintwork was deeply scored and scratched - the result of dirt and trapped sand particles.
In hand washing, invariably too little water is used. Even with a garden hose and a sponge technique, small sand particles lodge themselves firmly in the fine pores of the sponge or wash mitt and cannot be removed even by good rinsing. Such particles work on the paint surface like sanding discs. Furthermore, the hand washings produced a tangle of uneven scratches in the paint surface.
By comparison, the surfaces of the test vehicle washed with automatic car washing equipment appeared different. Compared with the hand washed surfaces, it was remarkably smooth, with many very fine markings virtually parallel and uniform - less than .0003mm - the result of evenly moving and rotating cloth pads and curtains. The machine-washed surfaces were in very good condition after 25 washes.

Wash a car by hand or by machine, in either case, it looks immaculate right after the wash. The sophisticated test described suggests that looks are deceiving: The paintwork on a car washed by automatic car washing equipment is in much better condition than one washed by hand - even though the car owner lavishes tender, loving care on their vehicle.
Eventually, either by force of habit or because the car finish looks dull, the weekend hand washer resorts to the lengthy and back-breaking chore of applying abrasive rubbing compound or similar pastes and liquids to their vehicle. This process indeed restores the luster to the finish. Such "pampered" cars naturally appear to have just rolled out of the showroom. The proud motorist forgets the long hours with the rubbing compound, and is probably unaware that the reason for all that work (to get the scratches out) was their energetic hand washing procedure on previous weekends.
On the other hand, the motorist who uses the services of a professional car wash with their automatic car washing equipment subjects the paintwork of their vehicle to the thorough, but gentle action of cloth pads and curtains which treat their paint job with tender, loving care. Knowing that the car finish has to have some protection against the ravages of the chemicals in the air and to also provide a barrier of protection against bird droppings and other contaminants, this car owner will periodically apply a coat of wax to their vehicle. At that moment, their car looks as if it had just rolled out of the showroom.
In comparison, although both cars after washing and other treatment will look pretty much the same, a car is actually treated better (especially concerning the paint finish) by machine washing at a professional car wash and the application of wax than a car washed by hand (with the resulting need for the application of rubbing compounds, etc.). However, in the process, the car owner who utilized the services of a professional car wash has saved a great deal of time, labor, and effort.