Many moons ago when I was young, foolish (and skint) I knew which garages I could take my Viva to that were renowned for "passing anything with a wheel on each corner". Indeed a well used trick within my circle of friends at the time was to cover rust holes in the chassis or inner wings with newspaper, underseal it, and when the underseal had set, remove the newspaper. The Manchester Evening News was the best as their paper was thicker and less likely to go to pieces during removal.
I think it's a lot harder for that sort of carry-on to go unnoticed nowadays.