When you plan to use a fusible (interfacing, underlining, or adhesive) on a recycled necktie, it is important to be sure the grainlines of the tie fabric are as straight as possible.  You can usually use the pattern in the tie as a reference - if the pattern is not distorted, the tie is not stretched.  I lay a tie out wrong side up on my ironing surface and adjust with fingers until there is no distortion, or until the fabric appears "squared" up.  Then I apply whatever fusible I am using before moving the tie again.