for race cars? yes you do. racers use solvents like brake cleaner that will erase a printed graphic like it was never there. the cars get greasy and grimy and the graphics need protection.
find 'cheap' lam, clear vinyl...something, anything. I've spray lam'd stuff for my own car when i first got my printer and it looked like crap within 2 weeks.
Oracal 3621/lam is the ideal combination for stock car stuff, imo(thanks outlaw graphics for this advice). even 'small' sponsor graphics are great with this because if the racer wants to move it, just peel it up and put it somewhere else on the car (yes it works, did it all the time).just clean the area and squeegee it down in the new location/on the replacement panel, etc.