White ink is treated as a spot color or dedicated layer in the file. The RIP maps it to the white channel and prints it in the correct order (underbase or spot). Setting it up wrong causes missing white, wrong order, or doubled white.
Spot vs process
Process (CMYK) does not include white. You add a fifth “spot” for white—a named swatch or a separate layer that the printer's RIP recognizes. Naming and color definition must match what the printer expects.
Layer order and trapping
White usually prints first for underbase, or last for spot white on top. Trapping (slight overlap or choke) avoids gaps or halos. Ask the printer for their preferred spot name and trap value.
Before you send the file
• Create a spot color/layer named as the printer specifies (e.g. “White,” “WHITE”).
• Place white where it should print (under color or on top).
• Do not flatten or merge white into CMYK; keep it separate.
Common mistake
Using a process white (e.g. 0,0,0,0) or a tint and expecting it to print as white ink. The RIP will not send that to the white channel. Use a proper spot white definition.