Knockout means the area is cut out of the layer below so only the top layer prints there. Overprint means the top layer prints on top of the layer below without cutting it out. For white under color, white must print first (no knockout of white by color); for spot white on top, white overprints color.
When white fails
If color is set to knockout white, the RIP may knock out the underbase where color sits, so color prints on the dark substrate and looks wrong. If white is set to knockout color where it should sit under color, the underbase can disappear.
Correct setup
Underbase: white prints first; color overprints white (or knocks out only where color is not printing). Spot white on top: white overprints color. Check your app's overprint preview and the printer's spec.
Checklist
• Underbase white: color overprints white, or follow printer's rule.
• Spot white on top: white set to overprint.
• Use Overprint Preview to confirm before export.
Common mistake
Leaving overprint at default so color knocks out the white underbase. The white disappears under the color and the print fails. Set overprint correctly for white and color.