I have done a lot of experiements with aging sheet metal, along with sword and knife blades. For sheet metal my best results have been with a quick "de-galvanizing" bath of regular house hold white vinegar. It will strip the zinc coating in a matter of minutes - and be careful - if you leave it in too long you will have heavy corrosion. I stupidly stripped the zinc off a shirt of chain mail by dunking it in a container of vinegar for a few hours, leaving it unnatended, only to return and find a 35 pound shirt of woven, rusty wire. It looked absolutely ruined. The cleanup required to remove the rust was mind boggling.
To further age the metal in a more controlled fashion you can fill a spray bottle with vinegar, dilluted with as much salt that the solution can hold. Spritz the metal surface daily without wiping off the excess, allowing it to sit overnight. Wipe off the surface rust every morning and continue applying the vinegar solution until the desired effect is reached. I have not found it necessary to do much in the way of abrading the surface with steel wool or sandpaper before the process begins, but a final buffing with 0000 steel wool leaves a nice natural finish. My final step in the aging process is to take a candle and burn the soot into all the pits and crannies of an object and buff off everything on the surface, leaving the corners and deepest areas with some grime in them.
Lemon, tomato, mayonnaise..... They all work to some degree as an aging agent. They all contain weak acids but plain old vinegar is cheaper and more effective. I have been told about a few home-remedies for darkening, aging, and protecting steel. I was given the recipe to boil young pine boughs in a large container of water, with a weak tea solution (presumeably for tannic acid?), along with a few small blocks of paraffin wax. Somehow this created a protective layer of patination with a waxy coating that was devised to protect steel animal traps from further corrosion when left outdoors. It worked well and made for a very authentic protective coating on my chain mail..... if just a little piney and waxy.