Several weeks ago I posted a video about an N scale corrugated metal siding maker for N scale. Here is a video showing how to weather corrugated siding made from aluminum foil in a way that is realistic.
I certainly recall, from years ago, how the old galvanized sheeting looked when years of weather had passed. Nice.
Diluting the etchant with distilled water slows the reaction making the timing a little less critical. You can also use used etchant that has been used to photo etch brass or PCB's which will slow the reaction as well.