Mottling usually refers to uneven/streaked colour appearance or poor metallic finishes due to uneven thickness of paint and poor distribution of silver granules, usually due to uneven spray pattern, over thinning, or spraying clear coat before the basecoat has dried.
Common Cause
Insufficient mixing of paint
Incorrectly thinner or thinner ratios
Unsuitable spray viscosity
Uneven spray thickness and poor spraying technique
Insufficient flash of times
Spraying to thick or too wet
Application temperature
Repair
Mottling can be repaired by re-applying your base coat with a medium wet coat, ensure your do not apply too heavy as this could result in the metallics grouping together.