Hang them out to dry, or allow a few days for some natural moisture to · dry. TIP: To clean the back of a number machine you can use rubbing alcohol, nail polish remover or a cotton bud. The problem is to figure out a proper way to wash the machine. What about the back of a paper book? If it's self-inking, use acetone. If it's a paper book, you can use a cotton swab for a few days. After that you can use a solvent, such as nail polish remover or isopropyl alcohol. Tips for self-inking: Clean the page from the inside, not the outside. Don't use nail polish remover on a self-inking paper book. Don't use acetone on a self-inking paper book.