Interested in Language
Does the sentence sound natural?
"He comes from France, but his English is perfect, no accent you could cut with a knife."
Thanks a lot.
Not a Teacher
"Invading armies have no rights." Noam Chomsky
No. You can describe a heavy, hard-to-understand accent as being so thick you could cut it with a knife. You wouldn't use that expression, even with a negative, to describe the absence of an accent.
I am not a teacher.
The only way I would combine the phrase with a negative would be in something like "Unlike many of his compatriots, he does not have an accent you could cut with a knife."
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.