There is an idiom in Indian languages :
There is no use washing the skin of a dead rat for even a year
The idiom means a foolish person or thing can not become useful even if we try to mend them for a long time
I would like to have an almost equal idiom in English
It is not a duplicate because the idiom considers to be equivalent to be mine does not even nearly mean the same.The meaning is totally different.
Washing the skin of a dead rat means trying to make something good for too long when the thing brought for mending is useless even in the begenning.It was really foolish to start it and continue to make it useful all the more foolish.
Best Answer
A very old saying comes to mind: "you can't make a silk purse out of a sow's ear" meaning it's very difficult to make a fine article out of inadequate material, or it's impossible to train a very stupid person to become the owner of a brilliant mind.