I had a long discussion with my friend, Jabid, about physical Bitcoin. He compared them to wine, or art.
Wine can "go bad" with time and bad storage (light, temperature, humidity,...). Do you buy fine wines to drink, or to keep? What do you do if a bottle is "corked" ("bouchonnée" in french)?
Works of art can be counterfeited. The provenance and history of the works are very important and experts are sometimes needed to authenticate a painting.
In the future, the provenance of OG physical bitcoin (like Casascius, Lealana, BTCC,...) might become important, and perhaps experts like minerjones or elianite might be needed. I wouldn't buy a Casascius on ebay.
That said, following teegumes' thread, I print the keys on special paper that I order from Novalith in France, and I use pigmented ink. I dipped the keys in water with rust and after several months the qr codes were still readable.
But as Smoothie says: coins are not hack proof.
My 2 sats
P.S. In the future I will probably consider doing 2 pieces for the priv key (
Krogoth)...