HDPE can also be repaired with various epoxies. Neat tip in this video about heating the repair area until water does not bead on it
http://www.youtube.com/watch?v=h-9yhanz5DE
I use Miller-Stephenson 907 epoxy on tanks, works well - even better with the heat/water trick. Just did a Tecumseh 5hp tank that something chewed a hole in last summer, patched over with another piece of HDPE.