How to write a true double negative in French
I'm trying to express a double negative, such as "It doesn't do _nothing_". In spoken English, the word "nothing" would be emphasized. This statement usually is said to contradict someone. It is sort of like saying, "Yes, actually, it does something!"

This doesn't work in French directly: "Cela ne fait rien" means "it does nothing."

How do I express the same sentiment. Can I use a double negative in some form, or should I just say, "Mais si, ça fait ..."

Question

How to write a true double negative in French
I'm trying to express a double negative, such as "It doesn't do _nothing_". In spoken English, the word "nothing" would be emphasized. This statement usually is said to contradict someone. It is sort of like saying, "Yes, actually, it does something!"

This doesn't work in French directly: "Cela ne fait rien" means "it does nothing."

How do I express the same sentiment. Can I use a double negative in some form, or should I just say, "Mais si, ça fait ..."

Answer

Une double négation en anglais a une valeur positive, c'est a dire: 
"It doesn't do nothing" veut dire en anglais "It does something.", mais cette phrase a un ton contradictoire.
"ça [ne] fait rien" toute les deux ont le meme sens - le 'ne' est simplement mois familier. Et viola la question: comment exprimer "It doesn't do nothing" ?

Answer

it does nothing  ....  ça fait rien ..   usuel mais pas trés correct
it doesn't nothing....ça NE fait rien    double négation 
???