Cliff notes version-
The cause
Particles produced from the friction of brake pads rubbing against the rotor, rail wheel friction on tracks or the metal scoop of a snow plough. This metal on metal friction disperses tiny particles of statically charged black dust, which adheres to the wheel and other vehicle surfaces. Sequestered brake dust, if left for any length of time will cause galvanic corrosion, which etches the paint surface allowing contaminants like acid rain, ozone, sulphuric acid, dioxide and nitric acids to penetrate the paint film surface.(See Industrial Pollution)
Once the clear coat is compromised UV radiation will further degrade the paint by causing oxidation.
The metallic brake dust acts as a conduit for industrial contaminants to permeate the paint film surface, that over time can cause permanent etching which, take the contaminants harmful effects (corrosion) right to the base of the paint system and down to the vehicles sheet metal.
Neutralizing -
Washing the paint film surface with a slightly alkaline car wash concentrate and thoroughly rinsing will neutralize the contaminants and remove them from the paints surface Detailer's clay removes imbedded brake dust and other contaminants by encapsulation, thereby eliminating the primary cause
Treatment-
By applying a suitable cleaner (Por-15® Products Marine Clean) and then a rust inhibitor (POR-15® Products) it will arrest corrosion on contact without leaving a sticky residue that would attract dirt, dust or grime. Corrosion attacks metal by an electro¬chemical (galvanic) reaction with water as the catalyst.
JonM