Making cards "shuffle-able"

I print my game cards onto cardstock with an inkjet. This works well for me, but the cards that come out don't always shuffle very well. I'm wondering what treatments people have found to make cards shuffle easily.

First, I am aware of the simple idea of placing them in card holders. However, I don't usually use 2.5" x 3.5" cards, but custom sizes, so card holders aren't really a good solution. I'm also aware of the approach of printing onto labels and sticking them to existing playing cards, but I don't do that because I usually need to be able to print on both sides of the cards.

A friend showed me some cards that he had sprayed with a fixative spray, something he got at an art store. I thought he said it was charcoal and drawing fixative, yet I bought a product by this name from Grumbacher, called "Matte Tufffilm Final Fixative", and it has no visible effect on the cardstock texture. This guy's cards seemed to have been "roughened" a bit, which made them easier to handle and shuffle.

So, does anyone know of a spray-on product or something similar that makes cards easier to shuffle? I haven't cut the sheets of cards yet, so it can be something that can treat a whole page, or just the individual cards.

Your assistance greatly appreciated!

-Jeff