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Reason #1: Water softeners and water spots
Manufacturers of dishwashing detergents assume dishes will be hand dried, and many detergents do not contain water softeners that will prevent spotting. Using dishwashing detergent on an automobile increases the likelihood of water spotting. A car will look shiny and clean at first, but the spots will appear soon after, much to the disappointment of the car owner.
However, most carwashing products contain ingredients that soften the water, which helps in preventing water spotting.
Reason #2: Grease cutters
Dishwashing detergents are generally made with certain surfactants which are designed for grease cutting and removal. However, these surfactants can strip off silicones and waxes on a vehicle’s surface. The surfactants will also strip any protection on bare metal parts of the vehicle, such as the engine compartment or the areas inside the doors. This can promote rusting when the detergent finds its way into these areas. Just as well, any chipped spots on the paint are susceptible to corrosion.
The end results are a dull, unprotected finish and the possibility of corrosion in undetected places.
Carwash shampoos are formulated with surfactants that are gentler to the car’s surface in that they do not strip the car of wax or sealant protectants applied to maintain a glossy and protected finish.
Reason #3: Emollients
Dishwashing detergents often contain cosmetic chemicals or emollients to keep hands soft and prevent the cracking and drying that can occur when immersed in water for too long. It’s nice for the skin, but not as nice for the car. These emollients will make a vehicle finish difficult to dry and/or leave it with a smeary appearance and dull finish.
Carwash shampoos do not contain emollients or anything that would be a cosmetic “perk.”
Reason #4: Surfactants
The surfactants found in dishwashing detergent can produce foam that is thicker, denser, and often will not rinse as well as traditional carwash shampoos, which can certainly be a huge problem and a waste of time while washing a car. On top of that, because dishwashing detergents may not completely rinse off, the vehicle may end up with water spotting or a smeared finish.
The surfactants added in carwash shampoos give a thicker, stronger appearance to the product, since “no foam” is often misconstrued as a weak shampoo. However, the surfactants used in carwash shampoos are not the same as surfactants used in dishwashing detergent and will rinse easier.
Reason #5: Dirt removal
For the most part, the surfactants used in dishwashing detergents are not the same as those used in carwash shampoos, as mentioned above. Surfactants used to remove food residue and grease from pots, pans and plates are not the most effective surfactants to remove dirt and oil film from a vehicle. The surfactants used in dishwashing detergents can leave dirt and oil film behind, causing you to work harder having to wash the vehicle more than once.
The surfactants used in carwash shampoos are specially formulated to remove both dirt and oil film.
Reason #6: Additives
Dishwashing detergents contain no special additives. However, well-formulated carwash shampoos contain special additives that provide benefits for hand carwashing. For example, a small amount of high pH additive helps to more effectively remove the dirt and road film and to neutralize any acid rain spots that are on the vehicle. These additives are not contained in dishwashing detergents.
Reason #7: Table salt
Sodium chloride (table salt) is commonly used as a thickening agent in dishwashing detergents to give them the appearance of a quality product. Washing a car with dishwashing detergent is literally like using salt water. This can result in severe water spotting and the promotion of corrosion.