The ink used for marking pressure treated lumber grades is a water based ink that will naturally disappear over time from rain and foot traffic. You can speed up the process by scrubbing it off with a bar of soap and a brillo pad. If the mark doesn't completely disappear you can sand it lightly with a palm sander using 150 grit sand paper. Some deck builders also use a paint thinner to remove faced stamps on wood.
Lumber Grade stamps can be erased with a little sanding.
