There are two main types of mottling that can occur in printing:
1. Optical mottling, usually seen with high brightness glossy papers, makes the paper look dirty due to unwanted variations in optical properties.
2. Water induced mottling occurs when water from previous printing units does not fully absorb into the coating before ink transfer, or when ink cannot fully emulsify surface water.
Backtrap mottling is caused by an uneven pore structure in the coating layer, resulting in variations in the thickness of set and unset ink that follows the coating profile. It is always present to some degree but usually well controlled.
