First of all check the fibre content of the jacket - if the corduroy is all cotton then fine - if it is a mixture with man made fibres then it could be problematic with the cotton taking the dye and the man made not, so it looks speckled.
Convert the jacket first, then wash and then dye it.

I used to do alterations on costumes and it was possible to see where things had been dyed and then altered afterwards that the dye had not penetrated evenly.

Anne the Pleater :ootd: