Every month I filter out my mineral spirits.
It’s a messy process but a needed one as the paints and mediums I use often get quite heavy over the 30 days - things can get a little messy. It’s a fairly straight forward process, taking a household paper towel as the actual filter I simply pour the old mineral spirits in and wait. After about 30 minutes it’s ready and since I use a two bucket system, the old filterered minueral spirits goes in one bucket and fresh mineral spirits into the second. Old sludge gets recycled at the local dump.
It certainly doesn’t filter it out perfectly clean but it does help manage the worst, making things last just a little while longer. Of course if I was so inclined I could use a better filter (or double up the paper towels), which I have tried before. While the results are better, it's not terribly time or cost efficient. Still, there's something satisfying about take the old and making it a little better, cleaner.