Fred, here is the method I use.

Flash each piece to be glued. Lightly sand each piece. Wipe with alcohol. Rub each piece with 3M adhesion promoter, these come in small packs just like alcohol swabs. Wait about 5 minutes to give the adhesion promoter time to dry. Then use CA about a 1/4 apart, DO NOT USE TOO MUCH, as this will only lenghten dry time.

Follow this method and I will guarantee the coroplast to hold. I have never had a faliure.

Kevin