http://theindianvegan.blogspot.com/2012/10/all-about-bilimbi.html
Locals make bilimbi candy and syrup by soaking the fruit in mineral lime overnight and then boiling in sugar several times over two or three days.
https://hort.purdue.edu/newcrop/morton/bilimbi.html
To reduce acidity, it may be first pricked and soaked in water overnight, or soaked in salted water for a shorter time; then it is boiled with much sugar to make a jam or an acid jelly.
It's compared with lemon which is also acidic.
Your post -
the fruit also useful to clean your nails, better than any artificial chemistry liquid in the world. My little sister's friend has started to produce a dish washing liquid using this fruit instead of the lime fruits.
https://hort.purdue.edu/newcrop/morton/bilimbi.html -
Very acid bilimbis are employed to clean the blade of a kris (dagger), and they serve as mordants in the preparation of an orange dye for silk fabrics. Bilimbi juice, because of its oxalic acid content, is useful for bleaching stains from the hands and rust from white cloth, and also tarnish from brass.
That's exactly what I did for my bilimbis but I skip the limewater soaking 😁 because I don't like it crunchy taste