Thinning putty

Is there a way to thin putty? I have a full tube of Squadron's Green putty that I bought 10 years ago (I haven't modeled that long). It's kinda thick and hard to spread.

I was wondering if anyone knew what kind of thinner to use. Thanks!

I'm not sure what's used to thin the putty when it's manufactured, but you should test some of it w/lacquer thinner or paint thinner, or even liquid cement (the kind made for styrene). I know that liquid styrene cement worked well for a friend using a Evercoat catalyzed (two-part, like Bondo) spot putty when it started to dry up inside the can before it's all used.

Buy yourself a new tube... Go to an auto supply store and get some automotive spot putty.
You can try thinning the old stuff but all you are going to do is cause problems down the road. I never liked the Squadron putty because it shrinks to much. Mixing it with other chemicals or thinners is not going to make it any better and may result in a lot of shrinkage AFTER the paint is done. It's not worth it...
For the past few years I have been using a two part filler called "Icing". It is a catalyzed polyester filler, doesn't shrink and sands very easily. Good stuff in my experience and much better then the run of the mill spot or glazing putty. That's what I recommend.