Marking Blue or Layout Stain is a dye used in metalworking to aid in marking out rough parts for further machining. It is used to stain or paint a metal object with a very thin layer of dye that can be scratched off using a scriber or other sharp instrument to reveal a bright, yet very narrow line in the metal underneath.
The advantages are that any existing scratches are covered with the dye and the new lines have a contrasting background.
Felt tipped, marker pens are a good alternative to the dye as they are convenient and tend not to dry up as quickly, marking blue in the form of dye or ink has a tendency to dry in the containers and become unusable quickly.
Other alternatives are chalk (used on rough castings) or a copper sulphate solution that leaves a deposit of copper on the clean metal. This thin copper coating is more resistant to rough handling and the action of coolants or cutting compounds