Have you got a few cans of old paint around the house? You know the ones, the paint is five years old and you have changed the color of the rooms twice since then. No one wants the liquid paint. So what do you do? Most places do not want liquid paint for disposal.
Well you could take the lid off and in another five years it would be hardened solid. Some places sell stuff to add to the paint to harden it so you can dispose of it more places. Trouble is they charge a lot for that stuff. My local trash people will take hardened paint only.
Well my trash company suggested clumping type kitty litter. Hey I have two cats, I have plenty of that around. One scoop and about a minute of stirring and I have a solid dry mass. Compared to the price of the stuff at the store this idea is free.