If you leave the drum uncovered, it will take longer to fill it.
Here we do have a time every year that home owners can drop off haz waste for free. That is just for home owners though. They also look closely at what you are bringing and it better be something that you would "legally" use in a residential area. I wouldn't take any automotive paint supplies there!
If I was a business, I would definately invest in an recycler for thinner. The stuff that is left is not considered hazardous, once it is dry, and you reuse the thinner that you have been buying.
Aaron
Hey.... If I remember right, Barry even bought one a few years ago, and we all know how tight he is with a buck. LOL