I use acetone to thin Squadron putty. I use two methods.
One is to apply the putty, work it into the areas that need to be filled, and then dip a cotton swab--one with a pressed paper shaft, not a plastic shaft--in acetone and use it to wipe away the excess. In doing so, wipe across the seam or gap, so as not to remove any of the putty, but to keep it in the seam. This also reduces the amount of sanding required. Here is a shot of a P-51 that I am building; I applied putty this way just last night:
The other is to put a blob of putty in a non-reactive (glass, ceramic or metal) vessel or container--I use a well in my ceramic palette--then add drops of acetone and stir with a toothpick, until you get the consistency you want. I call this "homemade Mr. Surfacer". I use an old brush to apply it to the area to be filled. As the putty dries out on or in your palette/plate/jar/shot glas, you can add more acetone to it, to keep the solution going. I use an eye dropper to add the liquid, in this case.
I must disagree, respectfully, with those who recommend nail polish remover. Yes, many brands use acetone, but they can include scents, and since they're typically dilute solutions, you're not getting as much acetone for your money as you would if you bought a can of it at the hardware store. If a small bottle of nail polish remover will last a long time, a bigger can of acetone in a more concentrated solution will last even longer. But, to each, his own, as it were.
Hope that helps!
Best regards,
Brad