"Tfmccarley" wrote in message
...
it will ride the surface like an air hockey puck with just the
lightest of nudges

It sounds to me like you are using too much blue compound.

"do no nudging whatsoever, there appear some number of dots on
the surface, but so faint that I often need a magnifying glass in good
light to see them"

or not enough blue. blue is the consistency of oil paint and would tend to
be sticky instead of allowing something to float. If your surface plate is
flat it doesn't much matter how much or in what direction you move it as
long as you move it through a the blue enough so the reading is effected by
any differences in how thick the blue is at various places on the plate.
3x6 is plenty big enough to scrape. I don't know why you'd expect dots
rather than smudges.

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. If you are
getting a circle of blue, either your plate has a high spot, or the blue
isn't even close to evenly covering the plate. don't weight it down, you're
just bending it to the shape of the plate. think about it - how is bending
the work to the plate going to show you what's high and low?

two things to consider 1) if the piece is so flimsy that you're concerned
about your grip distorting it (i.e., why are you using tweezers), what's the
point of scraping it? its going to bend in use or conform to what ever
shape its bolted to, and 2) this is a flat thin piece, there may be enough
internal stresses that its constantly changing shape, it least enough to
mess you up trying to scrape it down to a tenth of a thou (which may be the
case if you're putting the blue on so lightly you can barely see it). You
don't say what material it is, but I'd use cast iron if you have a choice.
What is the intent anyway for this piece??