bought new door panels from legendary, after a year for what ever reason the new felts didn't keep water out.
so there's warpage.
What can i put on cardboard after it dries?
a gel of some sort to harden it?
anyone?

Clear coat or maybe some sport of spray on wax coating? Does your car have the clear plastic sheet between the door frame and the door panel that is supposed to help prevent that?

Did you put the plastic liners between door panel and inner door skin similar to the factory. If not, you will be fighting this issue forever.

No i did not put plastic back, but i will when it goes back together.

repaired a set fore one of my 4 doers used water proof glue and those blue paper towels soaked in the glue kinda like fiberglass
to straiten and strengthen the edges by placing weights on them wile they dried used wax paper to keep things from sticking then used a can of flex steal to cote the cardboard and plastic sheeting

Are these the stamped cardboard ones or the vinyl covered?

I've made new backers for vinyl covered and transferred the upholstery to the new backers.