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 ..."