On a recent field trip to The Exploratorium, a cardboard exhibit was going up. I spoke with the curator about preserving cardboard structures for outdoor use. She said they should withstand a season's outdoor use by using the following preservatives:
-Bull's Eye shellac (about $30 for a gallon) to be used for colored cardboard because it's clear.
-Polyurethane
-Watered down Elmer's glue

I have had difficulty preserving art outdoors because of wind and water. Ideas?