Carduus

Greetings, science-types! I'm more of a literary type, specifically science fiction, but I like my fiction to at least have a seed of fact in it. This may be better placed in the chemistry forums as it's about grown crystals, but it's also about physical properties of geological elements, so...I don't know. I'm designing low-tech creatures on a very acidic planet who immerse themselves nightly in order to grow a crystalline crust to protect against acid erosion, and use some kind of file in the morning to sand themselves smooth. This requires a substance with the following properties: 1. Can be grown as a crystal or as an impure mix of crystals in a pool of chemical-tainted water over a period of 12 hours or less, possibly via electrolysis. 2. Is low enough on the mohs scale that filing it down to an even-thickness coat with a metal tool would be feasible. 3. Resists constant exposure to a pH 0-2 environment without eroding more mass in 24 hours than it would gain over the aforementioned 12 hours. Is there such a substance that exists? My research continually comes up blank.