The kid got a new electric toothbrush last month and I stuck the old one in my hobby desk and was considering converting it to a sander. When out of the Blue Inspiration hit while I was trying to clean some paint bottles from a few projects back that had enamal paint turning to sludge in them. After cleaning out the bottles as well as I could I filled them halfway with thinner and went at it cleaning the semi dried paint off the inside of the bottles… The rotating head on the electric toothbrush worked great and made short work of the job.
I have found the best way to clean paint airbrush bootles and lids to just drop them in the CSC for a couple of days. The paint come right off. I have a quart freezer container filled with CSC just for paint bottle. If the bottles have the setup sludge in the bottom I usually fish out what I can with a small stick and then drop the bottle in the CSC.