In article , Michael says...

imo you're making this too complicated, the principal simple, you are using
paint to indicate where one piece isn't flat by comparing it to a flat
piece. spread some blue on the plate, lightly grip the work by the edges
and move it around. The spread of blue will be heavier when you start which
is more aggressive and as you to get it closer to the finished state, the
lesser amount of blue left on the plate will show more detail.

Well, it is a *bit* complicated. But mostly 'feel.'

Like you say, the amount of blue on the surface plate
(and its consistency) really matters.

For starting out, when I am roughing a piece, I tend
to have the prussian blue fairly heavy on the plate
and also fairly sticky, or thick.

By heavy, I would mean a dab or two on the plate,
not cut with any oil, and spread around evenly by
gloved hand.

Then the part is pretty much just touched down, and
the real high spots get a pretty deep blue mark.

As time goes on and the part gets flatter and flatter,
I try to reduce the amount of marking medium and also
to thin it out a bit more with some wd-40 so the
consistency is more even and thin. Then I need to
move the part around more to get a decent mark, but
the mark is still much lighter and shows finer
detail than in the example above.

I've found that most of my mistakes in the beginning
were using the wrong degree of marking, ie. I would
be trying to spot very finely and accurately, when I
had a thousanth or two to move. Or, using too much
marking compound when the part is very flat. Then
it just shows up all blue, there's no contrast.

Jim

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