Chris in Canada wrote:
Don't throw away that ancient nail polish bottle with the glob of thick red "tar" in the bottom. Completely fill the bottle with nail polish remover, replace the lid tightly, shake well and leave overnight. In the morning pour out the contents and wash the brush and bottle with a good squirt of dishwashing liquid and very warm water, rinse well and let dry. Use it to re-store, in both senses of the word, those last bits of acrylic craft paint. Just scrape the leftover "gucky" paint into the clean bottle, add a few drops of water, mix in a bit using the brush and give it a good shake. Once recapped the paint will keep in this bottle for a very,very long time and even has it's own brush. This works for craft glue that has thickened as well.