+1 on the Oxalic acid. ^^^
We buy oxalic acid for wood bleach, leather bleach, and other household cleaning uses. Read the warnings, it's relatively safe, does work wonderfully for many cleaning tasks, but like all things - stupid can hurt.
Just google it, and find it pure, bulk, and cheap. 2 lbs should last a while.
I found some phosphoric acid metal prep for ferrous or Al. at HD. Another "read and heed". Its pretty safe. Chemicals don't scare me, but growing up as a wanna be mad scientist, I respect the heck out of most of em. Singularly, some can make you injured very quickly , and mixed wrong make you dead just as quick.
Here's but one example - Dont' heat galvanized with a torch. DEAD if you get too much zinc oxide fumes. Flue like symptoms that can take weeks to recover if it does not kill you.
More common for us will be mixing acid into water - OK. Just go very slow.
Mixing water into acid - BAD! Kerpoof! Depending on the acid, can be almost instantaneous.
Water gets so hot it turns to steam, rapidly expands and vilolently sprays you with strong acid. Bye bye clothes, skin, and eyesight...
Same with Sodium Hydroxide. Used to mix a liter of 5N solution every so often. Put on lab coat and face shield. Start with 700ml Ice water. slowly add 200 Grams hydroxide while stirring. Do it all in deep stainless sink with water running so you can dilute or dump if it goes runaway... Add cold water to bring to 1L solution. Let cool, and bottle. You start with ice water, and if you add too fast, can end up with boiling. Its the main ingredient in many drain cleaners, aka lye.
Guys, just cause it never gotcha before, dont mean it cant or wont... 10+ years in the lab, and one day I had a mix go runaway. Scared the poop out of me, cause it goes from a bit too reactive to dangerous out of control in seconds. Please be safe to keep all your skin and eyes intact.
Long as your aware and follow good safety measures, it's all good. One moment of stupid can leave scars for life. Just like driving a car or boat.
Goodnight and cheers.
Blaine