I was looking at a different product the other day, Its waterproof ply that is laminated on both sides. In white only. It was around $80 a sheet but there is no painting or sealing needed and it doesnt expand and flake. You need to get ot cut on a panel saw with a scriber blade.
If you are interested let me no and I'll pass on details. (Sydney only) I dont know where to get it anywhere else.
Peter