I used to be a member here, just decided to drop by and see what's going on. Big Steve Earle fan.

Those of us with roots in the southern (or mid-southern) US might have an easier time than Brits at interpreting American slang. I agree that it was a "shot coat of primer" (a single coat from a spray gun; quick and easy, doesn't have to be pretty), and then he "looked inside" to rebuild the engine, and maybe also to take out the seats for more carrying capacity (you don't need the room for passengers, you need it for cargo). The context makes it seem pretty obvious to me.

As for "copperhead," yes, it's a poisonous snake, but it's also something else. In the American Civil War, which still informs a lot of our American mythology, antiwar Northerners who advocated that the South be allowed to secede were called Copperheads. Most of them lived in or near border areas, like West Virginia and Kentucky, where sympathies were mixed, and which also happen to be areas where moonshining (distilling illegal liquor) was big. Copperheads were ostracized by other northerners, which added to their already strong sense of isolation and lawlessness. I always figured that Steve was using the name "Copperhead Road" to strengthen the Appalachian links in the story and underscore the suggestion that this was outlaw territory.