Currency exchange desks will refuse to change banknotes which are in any way damaged or defaced. Make sure that whatever cash you bring is crisp and clean.
Wrinkled bills, torn corners, ink on the bills, folded, worn, faded and other defacements will result in a rejection of these bills being accepted.
Be advised if you take change in US dollars or pounds in Thailand from vendors or other businesses, if these bills are damaged or defaced, you will not be able to use them.
If you plan on visiting Thailand, stop by your bank before departing and change all your bills for new crisp bills.