Hi, Tom, I just use my fiddle rosin on my psaltery bow. I'm allergic to some rosins, though, so you might want to check that before you buy one. I actually use a rather expensive rosin for my fiddle, and since it's already here, that's what I use, but I also have some cheaper ones that work just as well. My favorite for the psaltery is the rosin cake that came from Unicorn Strings! I don't know where they got it or the brand, but it's nice. It goes on easily and doesn't leave a mess.
I didn't answer your question ... sorry! Hard rosin is not a problem. Stroke the bow over the surface and it should load. If the rosin hasn't been used much, this may take some time, but you'll eventually get your bow working. By the way, don't touch the bow hair with your fingers ...
Thanks that worked fine