I asked if you were on board stock because it has a particular effect on metallic inks, especially silver. The heavier the board, the worse this issue usually is. It is caused by the soft vehicle needed to carry the metallic particles diving into the board, leaving a thin layer of metal particle on the surface, which chalks.
I would reccomend semi metallic for this process. Your ink provider can blend the 877 at around 60% with something like 430 gray, and you should have no problems.
The other reply, regarding adding varnish, will also help, but a heavier varnish like transparent white would be better as o.p. varnish is soft and will also want to dive into the board.
good luck.