Mechanical damage to the coating due to flying small stones/chippings (loose gravel, from example).
Stones are thrown onto the paintwork at varying levels of energy (velocity/size). Depending on the force of impact, the top layers and the lower paint layers underneath can be damaged right down to the substrate. At the points of damage, humidity will penetrate the coating and lead to corrosion creepback and flaking.
HOW TO AVOID
There is no total protection against stone chips. Areas most greatly at risk can be given preventive treatment of additional stone chip and underbody protection (Glasurit 1109-1240/4 Stone Chip and Underbody Protection black) and/or the use of Glasurit 522-111 Softface Additive (4:1 by vol. in topcoats and clearcoats). An extremely scratch-resistant clearcoat, such as Glasurit 923-447 HS VOC Clear scratch-resistant, offers protection against small stone chips.
Stone chips should be touched-up immediately. Refinish using the recommended undercoats and/or topcoats.