Another one of the questions I get. This one in December of 2020:

Is it legal to hand write the UN numbers on a black UN placard with a magic marker ?

My reply:

Yes. Maybe.

Per 49 CFR 172.338, if more than one of the identification number markings displayed on placards, orange panels, or white square-on-points is lost, damaged, or destroyed during transportation, the carrier shall replace all missing or damaged identification numbers as soon as practicable.

In such case the numbers may be entered by hand using an indelible marking material as long as it is legible.

So, it can only be done by the carrier (i.e., driver) and can only be done during transportation, i.e., while on the road.

I hope this helps. Please contact me with any other questions.