veedubber rapping greensand involves gently tapping the mould all
around as you will know.
if you are casting something box like all you get is a slightly larger
mould cavity and a mould which will pull out of the greensand.
cooling shrinkage of the casting will hide most of this.

fins however are a bugger.
if you rap for and aft along the fin then all is probably ok.
if you rap across the finning then sure as apples you will shear off
the sand between the fins.
if the finning goes in more than one direction you're sunk.

a better method might be to use resin moulding.
this involves a resin inpregnated sand and a heated aluminium or brass
mold. the sand is flung on to the gently heated mold with the heat
setting the resin binder (you use a silicon release spray on the
mold).
with just the right heating of the mold the set sand gradually grows
around the mold. when it appears thick enough you flip the mould over
and shake off the unset sand.

after you separate the impression from the mold the impression is
allowed to cool and the impression halves are held together and
supported in dry untreated sand. in use often the impression will
survive for up to 4 castings

the set sand impression in the one I've seen had immaculate fidelity
and was separated from the mold without any rapping.

This is something I've not done myself. it was demonstrated one
evening at a local teachers college by the instructor. they teach
manual arts teachers who in turn use the technique to teach high
school students live casting techniques. the kids cast and machine the
parts of a scale field cannon.

the boss at my old works has one his son cast and machined in pride of
place on his bookshelf at work :-)

Stealth Pilot