Did you record your original in letterbox mode? If so, there is a convoluted way of getting rid of your "thin" artifacts that does not compromise the picture with a blur...
Yes, there is another way. You can uprezz it and it will get rid of all or most of your artifacts. See my post at the top of the Technical Section. Although this "side effect" was not my intention initially, I found that the thin artifacts I was experiencing in the original footage magically disappeared in the uprezzed version.