The following information has not been reviewed or endorsed by any of the product manufacturers. Chemicals can be dangerous. Do this at your own risk.
Evapo-Rust is a popular rust remover. It costs about $30 / gallon in 2023. I read the MSDS (Material Safety Data Sheet) and I believe the active ingredient is 10% phosphoric acid.
Kleen Strip Concrete and Metal Prep:
Kleen Strip Concrete and Metal Prep is about $18 /gallon. It is 30 – 40 % phosphoric acid.
I mix 1 part Concrete and Metal Prep to 2 parts water to create a 10% solution. This makes the cost of the solution about $6 / gallon.
Milkstone Remover and Acid Rinse:
Milkstone Remover and Acid Rinse from Tractor Supply is about $18 / gallon. It is also approximately 30% phosphoric acid and has a nicer smell than Kleen Strip. I dilute it the same way.
This is a mild acid. If you put your and directly into it, you might feel a burn on an open cut but it won’t melt your skin.
You should still use gloves, eye protection, and adequate ventilation.
If you spill it on your concrete, it will probably clean it to a chalky white.
You can soak rusty parts for 4 to 24 hours. The acid won’t attack the metal so if you leave it in longer, it won’t hurt.
This rusty, antique caster had stuck bearings. After 45 minutes in Milkstone Remover, the bearings spun freely.
I mix it in a 5 gallon bucket with a Gamma Seal lid.
You can use it multiple times. If it gets super dirty or if you think it isn’t working well, you can toss it out and mix new.
Phosphoric Acid is the same thing that they put in soft drinks so dumping this down your drain is like dumping 30 times the quantity of soft drinks down the drain. It probably isn’t a good idea but it probably won’t cause a catastrophe if mixed with enough water.
Evapo-Rust recommends diluting it 10:1 with water and spraying it on your lawn as a fertilizer. The dissolved iron and phosphorous should be great for your lawn. Dumping it, undiluted on your lawn would be like over-fertilizing the lawn and may kill the grass.