There are many misconceptions in the aviation world about proper aircraft cleaning methods. The most common would be the idea that any common product can used to clean the surface effectively. While this is true for occasional cleaning if the surface is heavily soiled, it is detrimental to the painted surface over time.
Regardless of marketed labeling such as “non-hazardous, non-corrosive, environmentally friendly”, every cleaner is not safe for regular use. Additionally, truly safe cleaners/degreasers can ruin a finish if not properly diluted. For example, when used for washing autos the average degreaser should be diluted 1OZ to 1 Gallon. However, it must be stated, damage will result to your paint if used. Though this concentration may seem low, the chemists behind the companies have justified reasons. For example, surfactants & emulsifiers can leech into or bleed into porous type paints is applied at too high a concentration or if not rinsed off before drying takes place. Furthermore, many general degreasers return to a hard granular form if dried in improperly diluted concentrations. Such processes can do damage to the paint on many levels, both topical & intergranular.
Next would be a misconception not unique to aircraft but very prevalent regardless. It is common knowledge wiping off dirt without some sort of lubricating aid will result in marring & scratching. Water is a universal solvent but offers little in terms of lubrication. Natural inclination would lead one to believe that every cleaner/degreaser which feels slippery in the hands will provide adequate lubrication for washing. However, this does not prove to be true as there is a distinct difference between slip & lubrication i.e. motor oil. I personally, use a variety of waterless/rinseless products which do not feel slippery to the touch but provide exceptional lubrication on both painted & unpainted surfaces.
Why is lubrication important? The abrasive effect of dirt to a dry painted surface is evident with the naked eye. However, the same grinding can happen when a cleaner does not “lift” & lubricate the dirt particles so they can be wiped off. Furthermore, one can have an adequately lubricating cleaner & still inflict damage upon the painted surface. This is primarily done by the use of wash poles, even if the brush is soft. This happens on soiled aircraft when the washer applies pressure in an effort to scrub off the dirt or repeatedly scrubs the same area in a circular motion rather than a sweeping motion.
How does this affect the paint? In either case (properly lubricated or not) the end result is the dirt & debris will have a sand-paper effect. It can be observed by the appearance of fine & deep scratches as dirt particles share no uniformity. As this process repeats itself over time the severity of the deep scratches will become so severe they cannot be safely removed.
How to wash properly
The absolute best method to prevent paint damage is the use of a properly lubricating cleaner such as a dry wash, water-less wash or rinseless wash. I can provide demonstration of each technique & give resource links for properly vetted products. This requires only a spray bottle & the appropriate microfibers however, it is highly dependant upon the technique of the cleaner to do so properly & the microfibers must be washed properly after each cleaning.
Next method, less preferred is to use an appropriate car/aircraft wet wash in order to properly clean while providing adequate lubrication. A soft wash mitt may be used but proper methods must be used in order to prevent scratching such as pre-soaking & proper drying.