ferric chloride is widely used in sewage treatment because it readily oxidizes stuff, as it will do with any equipment that’s left near an open container. This is also not the type of brown stuff you can wash out of a surface it has seeped into. Thus, use a protected surface on your workspace or use a workspace you don’t mind staining. Wash your hands after use, and if any exposed skin comes into contact, wash it off immediately. FeCl is not hugely corrosive, but will cause damage if left open over a long period of time.
I personally use this method as it is tried and trusted, and it won’t let you down if you are paying attention. Perhaps the best part of this is that ferric chloride can be made at home, using some nails and pool acid. Scraps of iron or steel wool can also be used.
Just be sure to make it in a well ventalated area, far away from anything that can be corroded.