Copper is just a VERY reactive metal, and the acids on the surface
of our skin makes things even worse. The only thing I know of that
will work for a shoe is using very high quality clear coats. I do
not make much jewelry, but I do a lot of work in bronze and for the
last few years, sprayed copper on architectural pieces, antique cars
and some art items. (there is also a clear coat MADE for copper and
it’s alloys called Incralac (thinner based) and Syncrlac (water
based) used a lot on bronze sculptures that are outside. About a 15
years life and pretty easily removed with MEK solvent then re
applied, without damaging the patina it is protecting. It is also
waxed 1-2 times a year. Also for wax, the absolute best wax we have
found for our bronze sculpture is Leaberon. A bit difficult to find
but when I do, we buy it in large tins, about 1/2 gallon. Great wax
but expensive. Sculpt Neveau in Escondido, CA carries a lot of
material suitable for copper. I might suggest you give them a call
and see what they might suggest.
I have been very happy with a product from PAR, their catalyzed
clear coat urethane. It makes a pretty heavy coating, but seems to
work well on bright copper elements in the out doors. It is used a
lot on high end marine, polished items that can still be used, , i.
e. cleats used to tie the boats to a dock. It is VERY tough. It is
pretty dang expensive too, about $140.00 a quart in quart units
(less by the gallon and less still by the 5 gallons).
With ANY sealer you are applying, sharp edge, corners will always
have a far thinner coat of what ever coating you are using than the
flat areas, so if possible, always give edges a radius versed a
sharp, square corner. This thinning of any coarting on the sharp
edge is a problem in any materials and any fields, it is the nature
of things (and physics). Hope this helps a bit.