Because of circumstances, I have a nearly endless supply of old 4x8 voting signs made of coroplast. I also have 20 acres of hiking trails, off roading trails and a private gun range that I make signs for, so I use those old signs since they're just for me. I tried a bunch of different methods of painting the coroplast with not very much luck until I found that if I use Rustoleum's plastic primer first, then plastic paint over that, it seemed to last and not peel off.
But in my case, I could still see some of the screen printed lettering on the voting sign and didn't really like that, so now I just cover the entire sign in whatever vinyl color I want for the background, then layer the vinyl letters over that. Here is a recycled trail sign covered in brown 651 then white for the lettering.