Varnishing Metalpoing

I’m working on a mixed media metalpoint.  I’m using gold, copper, brass, bronze, silver and bismuth nibs to represent the hues of an oriole I’m depicting.  I do not want the base metals to tarnish; additionally, I’m framing the work without glazing.  For both those reasons I’ll be varnishing the piece, with either a very thinned layer of platina shellac (the benefits & drawbacks of which we’ve gone over on the forum, so I won’t belabor that issue) or B-72 (I know, a better choice, but I don’t like how “plasticy” it feels/looks), then following either isolating layer with an application of wax medium.  Will this be sufficient to seal off the image and inhibit tarnishing?

Thanks,
Koo Schadler