While laundering money is illegal, cleaning it isn't. According to an article in the New York Times magazine, flu germs can live on paper money for up to 12 days. That's pretty scary.
Be careful when you clean paper money – if you damage it enough, you won't be able to use it or turn it in at a bank for depositing or exchanging for new bills. The best way to clean bills is to find out what stained it, research the best removal cleaners, then spot-clean the bill to see what happens before trying to clean the entire bill. Reviewing how to clean stained paper money will help you get this process right.
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Consider also: How to Redeem Torn Paper Money
Call Your Bank First
Before you try to clean any paper currency, check with your bank or a store than exchanges currency to see if they will accept your bill as-is. They might tell you that you have to clean the bill, but only a certain percentage of it or a specific area so that it can be accepted.
The person you're talking to might also be able to give you information on cleaning agents, or have some of those cleaning agents in the bank or store. Ask what will happen if you remove any of the ink off the bill, tear it or make the stain worse.
Consider also: How to Cash a Torn Check
What Stained the Bill?
Try to find out what stained the bill. That will help you determine if it can even be cleaned, and if so, what type of cleaning solution you should use. For example, if you've spilled ink on a bill or someone has written on it with a Sharpie, that's probably a stain that won't come out.
If you've got some sticky food or dirt from the ground on it, that should come loose. Once you find out what cause the stain, you can Google possible remedies.
Consider also: How to Change Old Money for New Bills
What Cleaning Agents Will Work?
When you find out what causes the stain, perform a Google search using keywords that describe your specific problem. For example, you might search, "How to clean a smoothie off a dollar bill," or "How to get nail polish off paper money."
In addition to searching Google, try your keywords in YouTube. You might find some DIY videos made by people who had the same problem as you who figured out how to clean (and not clean) their money.
Wait to Use Cleaning Agents
Depending on how bad the stain is, you can try to remove the stain with a pencil eraser or water. Before you try cleaning the bill with soap or vinegar or any other agent in water, try cleaning a small part of the bill with water only. That will show you how the staining material will react once it's rehydrated.
It might run onto a clean area of the bill, making things worse. Start with a cotton swab, applying a few drops of water to an outside corner of the stained area. If you don't have a cotton swab, try a small piece of paper towel rolled up. Avoid starting with an abrasive material like a scrubber sponge or scouring pad.
Let the water sit for a while to see if it soaks in to the staining material and what that does. Next, gently rub the dampened area to see if the staining material starts to come out.
Spot-Clean the Bill
If straight water doesn't work, try using the cleaning agent you picked. Start cleaning the bill using your cotton swab or piece of paper towel dipped in your cleaning agent. Apply a drop or two to a clean corner of the bill to see what the solution does to the bill. You might let the bill dry before you confirm the results.
If your solution does no damage to the clean part of the bill, start cleaning the stained area, starting on an outside corner area. Let one or two drops of liquid sit for a few minutes to see if it does any damage to the bill. If not, continue to clean the stained area one small area at a time. If it looks like you are succeeding in cleaning the bill, continue.
Once you're done cleaning the bill, place it between two paper towels or cloth towels to help it dry faster. You can try a hair dryer, as well.
There's some disagreement about how to wash money in washing machines. Martha Stewart recommends cleaning batches of dollars that are dirty, but not necessarily stained, in the washing machine, using the gentle cycle. However, the New York Times magazine says that washing bills in a machine can damage security features, such as those used by currency sorting machines. Bills damaged by over-cleaning might be accepted for exchange by a bank, but the bank might shred the bills, or send them to be shredded.