Check washing is a somewhat unsophisticated but effective form of fraud in which a legitimate check's information is erased chemically or electronically, allowing a criminal to rewrite the amount of the check and the name of the payee. While there may be some defenses such as electronic inks and hidden watermarks to guard against such fraud, check washing works because many recipients accept the check at face value due to the legitimacy of the signature. Because the rudimentary method of check washing can easily damage a paper check, however, many con artists ruin more checks than they can cash.

The process of check washing is not especially difficult, although the results can vary widely. In a typical operation, a legitimate check is prepped for check washing by placing a protective seal over the signature line. This could be a low adhesive tape or sticker. The check is then held with tongs and placed in a pan usually containing acetone (nail polish remover), paint thinner or bleach. The chemical solution dissolves many types of ink found in standard ballpoint pens. Once the ink from the check has dissolved completely, the check is hung up to air dry. The result, ideally, is a signed blank check, which the con artist can rewrite to suit his or her needs.

One popular method of obtaining legitimate checks for check washing is to drive through neighborhoods and secretly inspect outgoing mail left in curbside mailboxes. Payroll checks and bill payments are especially popular targets among check washing enthusiasts. Sophisticated con artists have been known to carry portable computers, laminating machines, scanners and high-end printers in their vehicles which allow them to create false identifications in order to cash batches of washed checks.

The main problem with the check washing scam is creating a truly blank check. The ink contained in a standard blue ballpoint pen is easily removed with acetone, but black ink can be problematic. Experts say gel pens with black ink provide the best protection against check washing, since the gel ink resists chemical stripping and contains pigments which permeate the fibers of the check itself. Check washing is not profitable for the con artist if the checks look altered or bleached out.

One way to protect yourself against the threat of check washing is to switch from blue to black ink when writing checks, and to use a gel pen whenever possible. Additional safety measures include taking your bills and other outgoing mail containing checks directly to a post office or public mailbox. A personal mailbox with a lock may also discourage outsiders from stealing your mail, but you may not be able to use the mailbox for outgoing letters. Invest in checks containing anti-fraud elements such as electronic inks, hidden watermarks or microprinted lines which cannot be photocopied or scanned clearly.