Main cause: Incorrect initial cleaning or other surface preparation. If sanding dust or other dirt has not been completely removed, the topcoat cannot firmly adhere to the primer, leading to poor adhesion.
Incompatibility between the primer and topcoat or between the primer and the substrate.
The topcoat is sprayed before the primer has dried.
Insufficient sanding or lack of sanding altogether makes the surface too smooth.
Low-quality thinner and its poor solvency.
Topcoats are applied before the first coat has dried, when painting with more than one color.
When painting with metallic paint, insufficient evaporation time between two layers or too high viscosity.
Too long a time gap between applying the primer and topcoat.
An invisible water film is present on the primer surface before spraying the topcoat:
The weather is too humid when applying the primer.
The primer surface is left to dry too long during a damp night.
Solution
Remove the existing paint by sanding and repaint.