Smudges sometimes appear on the finished card after the card has been handled in the USPS sorting facilities. Although out of our control, our printer has spent a great deal of time and energy working with USPS facilities to resolve this issue. They have researched different kinds of paper, printing equipment, ink and coating options to see if making changes to current equipment would change the end result. They've also spent time with the USPS sorting facilities managers in an effort to improve the situation.
When these cards move through different sorting facilities, they are exposed to ultra high-speed sorting belts that rub up against the front side (non address side) of the cards. (see video link below) If these cards get exposed to the belts for even a millisecond longer than they're supposed to, they're at risk of getting abrasions. Sometimes, using a thicker paper stock actually makes the problem worse. This is a very unfortunate reality of mail in the United States and not limited to our printer and Thanks.io but spread across all printers and companies using direct mail post cards.
The good news is it doesn't happen very often! PLEASE NOTE: If you are looking at cards that have been returned to you, these cards have gone thru twice as many sorting belts (which is probably why you are reading this article and not happy). The postcard going straight thru to the recipient is going to have far less smudges if any at all, so please keep this in mind when reviewing any returned cards.
Here's what you can do to minimize the smudges showing:
If you use lighter background colors, the smudges will be less noticeable (eg. black is the worst and white is the best)
While the smudges are not always going to be isolated to a particular area, it does seem that the primary spots the belts hit on 4x6 size are center-left.