Best to route the sign and then print it. If you are using a flatbed UV printer, you can print a jig (outline) of the sign on to the bed of the printer and then use that to position the routed sign. Ideally, as long as your flatbed is large enough, you would set the sign up with bleed and the jig to the same size. As far as getting the ink to adhere there are a number of adhesion promoters - like Solv S1000. You can also coat the ACM in vinyl and then print on the vinyl if need be (great way to save a piece of ACM if your printer messes up).