The problem lays in the way you are going around the problem.
I would like to quote you back if I may.
any ideas on how to talk the wife into letting me paint the wifes car from this:
<Insert pic of daggy car here>
to this:
<Insert pic of cool GB car here>
The problem is you are trying to convince her.
You need to be a man and just do it and to hell with the consequences. Just say,