For a single word, one could use denude and it's various forms.
1. trans. To make naked or bare; to strip of clothing or covering; spec. in Geol. of natural agencies: To lay bare (a rock or formation) by the removal of that which lies above it.
However, if you were to take your car into a garage and ask them to denude it, I suspect they'd all glance at the Pirelli calendar hanging on the office wall before taking a deep breath in through their teeth and saying Could be quite costly.
Another possibility is strip
11. a. To remove (an adhering covering of skin, bark, lead, paper, etc.); to pull off (leaves, fruit) from a tree, etc.; to remove (paint or varnish) from woodwork, etc. Also to strip off. Cf. stripped ppl. a. b.
However, if you were to ask your local garage to strip your car, they would immediately set about removing any part of the car than can be removed, leaving you with a large pile of parts and a car body that is still completely covered in paint.
If we are not restricted to a single word then bare metalled seems to be term used occasionally in the body shop community.
The only term, that I'm aware of, that is used by body shop people for removing paint from a car is to take it down/back to the bare-metal. This usually involves sand blasting the coverings from the metal (glass beads these days for health and safety reasons). Some images can be found here http://the2cvshop.co.uk/shotblasting_1_classic.html
With thanks to Janus Bahs Jacquet, it seems that bare-metal can be verbed into bare metalled.
[SPR Coachworks Ltd] at https://www.facebook.com/SPRCoachworksLtd/posts/683435985028935
Porsche 356A has now been bare metalled.
We then applied an Epoxy coating to protect the bare metal from corroding.
[DasRestohaus] at http://dasrestohaus.com.au/gallery2/v/Type+2/Jacks+59+11+window/Oct29+nose+bare+metalled.JPG.html
Oct29 nose bare metalled
The nose was taken back to bare metal to do some repairs (yep even new panels are not perfectly straight)....deoxidine applied first to condition metal
Once you have it at the bare-metal stage, the next stage is to prime it, so you could say that the car is unprimed.
4. concr. a. The substance or mixture used by painters for the preparatory coat. b. A coat or layer of the substance. Also fig.
Humorously (or humourlessly) you could perhaps coin DeLoreanated with reference to the DeLorean DMC-12 which has a completely unpainted body shell