How do I make sure the hidden message stays hidden?

The scratch layer has to be fully opaque so the prize or code underneath never shows through until someone actually scratches it. Opacity comes down to how thick the coating is and how you set up the artwork sitting beneath it.

• Hold a printed proof up to bright daylight, then under fluorescent light. If you can read even a faint shadow of the message, the coating is too thin.
• Keep the hidden text high contrast, dark text on a light background, so a thin spot is far less likely to give it away.
• Avoid large solid dark blocks directly under the scratch panel. Busy or dark backgrounds telegraph the shape of the letters underneath.
• Ask for a heavier scratch coat or a silver panel rather than gold, since silver tends to block light more effectively.
• Always judge opacity on a physical sample from the same run. A screen mockup can never show you how much light the real coating blocks.