Pretty much every manufacturer is going to tell you that the steel will have to be painted before installing any sort of wrap film. That means sanding, prepping, prepping again when the weather doesn't cooperate, and painting. Then (probably) doing it again because the weather wasn't cooperating! And then wrapping.
I'd tell them they would be better served by doing paint only, but- if they insist on wrapping with vinyl- insist that you have to follow the manufacturer's recommendations to take the job, then stick to your guns. To do any less increases your chances of financial and reputational disaster.
As for the warranty- thoroughly examine the manufacturer's instructions and material warranty to determine what you're willing to offer your customer. I would be hesitant to offer any, other than for install errors (wrinkles, bubbles, cuts, whatever) and far less- if any- on the material than the manufacturer does (another reason to thoroughly explore the manufacturer's spec sheets regarding exposure and film life).
