Check washing is a form of fraud which involves stealing checks, removing ink written on them, then rewriting them to pay to the order of oneself.

Check washers often steal checks by prying apartment mailboxes open or prowling wealthy neighborhoods and opening mailboxes that have their red flag raised.

Once the stolen checks are in their possession, check washers photocopy the original writing on the check. Then they use household cleaners such as acetone or bleach in specific amounts with water to create a soaking solution, so that writing but not printed words wash off the check. Check washers then blot excess moisture on a bed of papertowels. Next, a hairdryer or iron removes wrinkles.

When the checks are dry, check washers write out the checks to themselves and trace the photocopied signature onto the washed check. They might also write something inconspicuous like carpet installation on the NOTES line.

The entire home process takes about thirty minutes. Check washers then cash the checks, often as themselves.

Some preventive measures are in effect such as pre-treated checks that make the word VOID appear upon chemical treatment of a check.

Sources:
Justice Files, Discovery Channel, airdate: February 12, 2001, 11 p.m. EST
http://www.ckfraud.org/washing.html