On Sunday, May 7, 2017 at 7:09:10 PM UTC-4, Andy wrote:
I have a 2 x 4' piece of underlayment lumber.

Any way to flatten it ?

Thanks,
Andy

"underlayment lumber" What's that?

When I hear "lumber", I think of a board. (2x4, 2x8, 4x4, etc) When I hear
"underlayment" I think of sheetgoods, e.g. plywood (luan, perhaps), particle
board, maybe even MDF depending on the application.

What type of product are you asking about? What are you going to do with
it/why do you need to flatten it?

A piece of luan underlayment could be "flattened" by attaching it to
a frame made of "lumber". ;-)