|Tip and photo supplied by Larry White:
On reverse glass panels, large areas were left open. A mixture of shellac and india ink was brushed or "airbrushed" into the area. After the area was dry, it was backed up with a single solid color.
Shellac can be used in this process to grab and hold fine airbrush spray. Without the shellac, the next layer of size or paint would lift the fine particles. Once dry, the area can be backed up with about any paint or leaf.
Larry backed up the blended areas on this sign with a variety of colors and types of leaf.