I used 1/2" ply on my exped and had a problem with one of the forms breaking near the end of the boat. A form where there wasn't a lot of material between the beam and the outside. Beefier ply would prevent this. On the Night heron i have started now i am using 3/4" MDF it seems to hold a staple (but not as good as ply) but I haven't started stripping yet. I chose MDF because it was easier to bring to the correct shape. A sureform or regular file fairs it nicely where as ply is a bit harder to fix. I am still debating on wether or not to indulge in staples so MDFs holding power might not be a concern for me.

Tom

: I used cheap particle board on the forms for my Spring Run. It was free. If
: you coat the edges with titebond it holds staples pretty well.