hmmm... haven't seen them mentioned for weathering, but they work great for thinning enamels and cleaning out airbrushes after spraying the enamels.
You can get them in the hardware store, look in the paint section. There's two types just pain Mineral Spirits and Low Oder Mineral Spirits. Not sure how well the low oder one works as I don't use mineral spirits myself as I spray mostly acrylics... I am sure someone will stop by and enlighten ya on that one in a few!!!
Oh and the what are they - they are a paint solvent, used for thinning enamel house paint and cleaning brushes with.. and obviously model enamels as well!!!