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In article , Jordan wrote:
Is there an advantage in using traditional marking blue?
Seems a lot more fiddly than a marking pen.

There are two kinds of marking blue:

There is a grease type like Prussian blue that is used for scraping and
precision fitting that will transfer from one part to the next.
Apparently Prussian blue is messy & very hard to clean up. I suspect
there may be better options. I've never used Prussian blue, but have a
home brew equivalent which is lamp black mixed with vaseline.

The other kind of marking blue is more like what you would get with a
marking pen, i.e. a thin dark material you can easily see a scribe mark
in. A wide tip black marking pen works fine for this, and is easy to
clean up with alcohol. I've used brush-on Dykem "layout fluid", but it
tends to go on thick, it can chip, and it's harder to remove than a
marker. If you have a large area you need to cover, you can get the best
of both worlds by using a Dykem felt tip applicator, which is like a
marker, but lays down a 1/2" wide stripe. It goes on thinner than the
brush on stuff, so it dries faster and is less likely to chip. It's
still harder to remove than marker.

Doug White