I have an old bottle of Roughing Fluid and it has become a little too thick. Can anyone suggest what would be the best liquid to add to it to thin it down a bit.

It may be alcohol that is used as the solvent.

Or just use the spray, the magic secrets area has an article that includes tips on roughing spray and how to buy it your local hardware/home improvement store.

If it becomes a losing battle.... Try some Testors Dull Cote, available in craft or hobby stores all over. You can spray it on.

I have used Krylon Matte Finish spray very effectively. I use it to make invisible decks. The secret is to use light sprays, let it dry, then apply additional coats as needed. Once I reassemble the decks, I riffle them (as if showing a svengali deck) several times from both ends, getting rid of any loose, powdery residue.