We have a vinyl wallpapered RV shower and I want to coat it with EPDM. The wallpaper is adhered to thin plywood and will not peel off. I successfully painted other parts of the RV wallpaper with Kilz and paint. Also, I purchased the Pro-Flex primer. So, will I need to use the primer and will the EPDM work in a shower? I don't intend to use it on the shower pan.