I always keep a can of solvent. When it gets a bit stiff I just add a bit. As long as I keep the lid tight it will last forever. The solvent is a bit harder to find. Luckily my local lumber supplier carries it. Some of the colors don't match perfectly mainly because every piece of wood is a different color. I have the hardest time with a real light maple. I keep red oak, maple, alder, walnut, and natural. A lot of times I use alder fill to fill oak.