Oxalic acid

500 g oxalic acid - at least 99.6% (crystalline)
in a can
For the solution of surface rust on porcelain, glass, ceramics and iron metal
Not suitable for removing sintered iron stains on porcelain / ceramics.
The concentration should not be less than 10 percent (1 part oxalic acid to 9 parts warm water) - depending on the rust intensity, an exposure time of a few hours to several weeks can be expected.
Brass can develop a reddish color; Stainless steel can turn gray.