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cyccommute is a chemical expert, and he's right that a jar of mineral spirits is the most efficient by many measures.
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robertorolfo, I dump my used mineral spirits on the ground or the road. There might be better places, and I'm open to hearing about them, but it's rarely more than a pint, and I don't do it often at all. We all dispose of crap consciously or not. I even have to think about what's going on when I flush my toilet, since we have a limited-capacity septic system. There's no such thing as being totally clean, only as clean as you can be with reasonable effort. My once-a-year dumping surely has less impact on the environment than driving a car a mile.