[vettestr]

Patrick,
Your mention of the corrugation is right on but those are like 3/16" thick, there are pure plastic (of some type) material signs that are much thinner and thus easy to do smooth cures with.

In spots you need to use tape or whatever works try spraying surface with PVA or polyvinyl acetate. PVA is water soluble so clean up is easy or hit it with high pressure air gun (but that makes shop clean up instead of object clean up). Have fun