What is the size of these panels ?? That's what will really determine if you want to clean them or not.
If you have the technology to digitally print and bid this job out, why didn't you think about how to prepare for your quoted job. If you had figured on removing vinyl and now have to entertain the thought of removing painted letters and sprayed background, you certainly are not in a position to strip anything and still make money [profit] in this job.
Your best choice is... to go back to your customer and tell them you screwed up. You didn't know these were painted and give them a choice of new panels or giving their deposit back. Anything else is going to end in disaster and I'm afraid a trip to the 'Emergency Room' with inhalation of toxic fumes if you don't have a proper space to strip signs, let alone the proper equipment. I will bet you an American dollar, you'll get sick striping these and then be asking how to do this process.
Why do people do the dumbest things and then wonder why everything goes wrong, when they're not qualified to do the most simplest of tasks ??