I was surprised to see Milliput (extra fine white) on the shelves of the LHS when I was buying other finishing supplies. I decided to pick some up and give it a whirl. I noticed when I took the bag off the "colored" stick that it had a significant 'skin' on it. Mixing the 2 parts was difficult at best, but I applied it to the 41 Willys for some sink mark fill in. The instructions say 3 hour hardening time, but after a day, the stuff was only crusty on the surface and gummy underneath. Can this apparently old batch be saved?