Digital BW, The Print

Hello,

I make my BO prints using MIS Eboni MK on Canson Rag Photographique and I'm very happy with the results except a rub-off problem. 
When I'm sure my prints are dried, I store them in a box. And after a few days of manipulations, some black marks appears on the back of my prints because they rub each other.


Do you think it's a normal issue ?  Using a spray could solve the problem ? 

Thank you

Benoit