How to protect cardboard interior panels from water damage

[fred93]

I am getting close to working on the interior installation. I have new cardboard side panels that will need to have fabric applied to them. What is the best way to make them waterproof from the inside? I looked at the parts that are used to install the rear side glass on my Tudor and I visualized the amount of water that will flow between the body and the glass since there are no rubber seals at that point. (someone told me that he uses pieces of round foam insulation at that point whenever he washes his car--great idea but what about when I take the car on the road and it rains????)

I know that it is recommended to install a sheet of plastic on the back of the panels under the windows to keep the water from staining the cardboard.

Is that good enough????

I want a EXTRA way to prevent water damage---Any ideas???