How to flatten warped hardboard

[Freddyjersey2016]

I am making a pine frame to support a piece of 4mm hardboard approx 1800mm x 600mm
- the hardboard has had a 'picture' painted on the smooth side
- it has then been left in a damp garage and has a distinct cup across the width
- the plan is to fit a hockeystick pine moulding around the 4 sides to restrain it - using panel pins through the moulding & hardboard into the pine frame

What's the best way to flatten it, before fixing it to the frame? - or do I just nail it down and hope!

 

[deema]

Buy a new piece and leave the other to dry out somewhere where its dry and can be flat.

 

[novocaine]

depends what you are calling hardboard, but assuming it's a basiically MDF with a plastic film lamination over it then it doesn't really contract and expand like normal timber, the warp is one side being damp and the other dry along with being stored stood up with no frame holding it. glue it to a frame to pull it straight if you can or leave it inside on the floor for a few days.

 

[Adam9453]

Its only 4mm thick so should be pretty flexible anyway.
I'd make a decent sturdy frame and just crack on.
You could try painting the reverse of the hardboard to balance out the paint on the other side but I don't really think its necessary.