Scotch guarding cardboard? Hmmmm... that's a first for me. From my own experience that stuff lasts as long as the material its applied to so it can vary with heat, light (UV) and friction... not to mention the amount and duration of the water.
Test a small piece and put it through the ringer of simulated rain and see how it lasts.
Another option is plastic wrap around the cardboard and using waterproof tape to seal the exposed edges. The problem with this is that some water can still get in. Check frequently to make sure the cardboard isn't rotting from trapped moisture.
You can also find coroplast online through some shops like this:
COROPLAST SHEET - COROPLAST - Order Online