OK, here's my way. True up the edges and run plain old masking tape the full length of the joint. Flip over and open the joint slightly, the apply hardware store Du Pont "Duco" Household Cement. Lay the sheet back down flat and cover with wax paper if doing multiple sheets. Weight the sheets down with books or similar. Let them dry overnight, then remove the tape and bond the sheets to the cores with epoxy or contact cement. The joints will be almost invisible and sand VERY easily.