"Difficult to erase" toner for laser printers, or ink for inkjets

Hi there,

Recently someone intercepted a check (cheque?) that was sent from our company through the post. The person who intercepted the check washed the dollar amount and the "payable to:" name off, and put their own name and a different dollar amount in. Luckily, when they cashed it our Accounting Dept. noticed something was wrong, so the culprit didn't get the $30k he thought he could.

It's brought to light the concern that this could happen more often, and I've been asked to research "uneraseable" toner for laser printers, or ink for inkjets. All I've been able to find so far is the security ink used by the banks to print the check, but that's no good to me.

Would anyone have any suggestions on this sort of thing?

The simplest step is to use paper that allows better penetration of ink or toner.
Much glossy preprinted stock lets the ink or toner sit on top of the surface where it is easily removed by scraping and/or bleaching.
A finely patterned background that will be damaged when any overprinting is removed is also a help.
Unfortunately, I expect that your cheques are printed for you by the bank.

Lasers printers are in general more permanent than inkjets as they are waterproof.
Some inkjet inks are water based and hence can be washed off, others are not.
an example below:
http://www.inkkeepers.com/product_details.asp?id=36900

The only other way to go is to use colour printing.
Colour lasers print at higher temperatures than mono ones, they also put on 4 layers of toner, and some now use organic toners which include wax. all of these factors make it harder to remove the impression by scratching.

Or, you could print your cheques in a particular shade of green for example, which would be difficult to match or replicate, and would show up when processing.

Dye sublimation printers can be even more permanent, as they actually dye the paper, but they are generally expensive.

Dot matrix printers may be another option.