Bowed sheets of DT Foamboard

I bought about ten sheets of dollar tree foamboard and I noticed that most of the sheets are bowed, maybe about 1/2 inch on each end if flat on a table. It isn't a problem if I am bending and folding the foamboard or using it in small sections but I was wondering if it happens with anyone else's foamboard or did I just get a few bad sheets?
 

Joker 53150

Mmmmmmm, balsa.
Mentor
I've also noticed that there is a difference between batches of the foamboard. I picked up some boards a few days ago to build a flying wing. Plans for the wing call for the paper to be removed and they show how to soak it to get it loose. Turns out the new boards let me simply peel the paper right off easily with no residue. I tried that with some boards I picked up a few months ago and there is no way the paper was coming off without a soaking first.

Strange, but for $1.00 I guess I can't expect perfect quality control from batch to batch...