Yep, it is sprayed down. I think what goldbacks do is ingenious, but i still do not seriously like it. If people start recognizing and accepting it, i will change my mind.

First line of defense will be these kind of metal detectors that are really focussed on gold.
Then it will be all like what we do with large denomination dollars. We use special paper, special ink, and other identifiers.

But, someone may making a scanning xray machine specifically for gold. That tallies up how much gold atoms it sees. Or something like that.