I collect the used mineral spirits in a jar and let the primer settle out so I can recycle the mineral spirits and use it to clean the brushes again.
I've found that instead of the mineral spirits being clear after the primer settles to the bottom of the jar, it is sort of an amber colored clear. I don't think it's a matter of not waiting long enough because I've gone weeks without touching the jar.
The recycled spirits seem to work just as well as new spirits for cleaning brushes, but I'm curious as to why it is no longer clear. What is in the primer that would cause the spirits to turn amber?