Exactly!
Mike (Grenadier1758) is correct and Mayo will also work, and so does Ketchup, and Vinegar. BUT...., when I started to try this, it was 15 or so years ago and my son was 3, and his neighborhood name was
"Danger Boy". He had no fear of anything, and was adept at getting into all sorts of stuff, anywhere it was stored. (I first started doing it to age knife blades from a factory polish.)
So caustic, proper browning agents were out.
Poison Control was already well versed on my boy...,
Mayo goes rancid, and would give the kid food poisoning.
Ketchup has sugar and draws ants (and they are crafty little beggars at getting paces too)
Vinegar beads up and gives the metal "measles".
Mustard he likes but he didn't recognize it when dry and brown, and so a barrel hanging in the basement smeared with the stuff was relatively safe, and he was too if he messed with it, or got into the gun-mustard-jar.
During the same time period my son demonstrated that
Soft-Scrub without Bleach is rather safe, but will make a kid very "regular"....
The blue hockey-puck in the blister pack in the bathroom that goes into the toilet water tank does NOT taste good....but does stain your lips very blue, and you spit it out...
The folding aluminum document holders for the tops of manila file folders...the kind that go through two holes punched in the top of a document and then fold over....well they have a thin coating that catches fire before the circuit breaker trips when you jam both ends into a live power socket....
A small sandwich WILL fit in the tray that loads VHS tapes into the VHS player, but it does not come back out, intact....
A 3.5" floppy disc must be removed from the computer without torquing the portion remaining in the A-Drive, or it breaks the A-Drive and that component must be replace on the home computer (twice)...
Red (or any color) finger nail polish on carpet is forever...,
Sorry...., we were talking about the finish on gun barrels and locks...
LD