White layer.

StickerGiant describes it as an additional layer of white ink printed behind the design to make chosen areas completely opaque, and says that without it even coloured areas on a clear sticker appear transparent. Its file setup is a separate layer, named White Ink Layer, holding a copy of the artwork turned white at 100% opacity; areas left empty, or set to 0%, let the clear material show through. Sticker Mule works the other way: all of its custom clear stickers are printed with white ink behind the design, and a white-ink-only sticker may still be slightly translucent on a dark surface. Who builds the layer therefore differs by shop, and the same transparent PNG can come out opaque at one and see-through at another. On holographic stock Redbubble says there is no white underlay at all. In UV-DTF, PNW Print Co says its RIP makes the white layer slightly smaller than the design, the choke, so no white outline shows around the colour. Ask whether the printer generates the layer or expects it in the file before drawing it.