Been using the same mix of ferric nitrate and water for twenty years. I know exactly how it will take on a given piece of steel, how the humidity matters, how clean the surface is, whether I've overheated the metal before applying it. The bottle from the supply house gives you a predictable brown that looks like nothing I've ever seen on actual weather-worn steel.

Last week I watched a younger guy in the shared shop spray on a commercial patina from a pump bottle and wait for it to look like something. That's not how it works. You build it in layers, you react to what the metal does, you stop when it's right, sometimes ten minutes in, sometimes three days.

I'm not saying everyone should mix their own. I'm saying the shortcuts don't teach you what's happening.