Pernambuco is a tropical hardwood, like Mahogany and Rosewood, and
as such the pores are very large. If the pores aren't filled, the
final varnish will collapse into them, and the resulting finish
will be unsatisfactory. Thus, French Polishing, using a muneca and
pumice, is quite necessary when finishing with these woods, as one
would do when bowmaking, as well as guitarmaking when these woods
are traditionally used for sides and backs. As was stated, Maple
and Spruce do not have these large pores, so pounding the wood
down with pumice/oil/shellac is pointless, and may harm the sound
of the instrument.
I think the big problem with French Polishing violins stems from
the practice of placing shellac over the original finish and then
polishing the shellac to a bright shine. The practice was
widespread because it was an easy way to make an old violin
look new but obviously destroyed the original finish of the
It's true that many modern guitarmakers fill these tropical wood
pores with epoxy, then varnish over the epoxy with nitro-cellulose
lacquer (usually), but a few traditionalists insist the French
Polishing is the only acceptable way to varnish a classical guitar.
They suggest that the sound is much better, although the finish is
quite fragile and must be restored periodically. The advantage is
that the varnish on a French Polished guitar can be quite easily
repaired if the luthier is skilled at French Polishing.