Sheeting foam cores
I do as the above poster mentioned,
Tape the sheets together, weigh it all down
Let them cure for 24-48 hrs, remove from shucks, remove tape from seems, The leakage through the seams is very small, and sands off nicely.
I think the trick to the small amount of probond seeping through the seams, is having a very tight fit to begin with. Edge sand all your sheets perfectly straight, and you will have no problems