Doreen, the headliner will show any imperfections of what is under it, plus not much of an insulating factor. Reflectix is an aluminized bubble wrap type insulation that should be glued to the the interior first (no need to get all the old glue off, just anything loose) with all the seams covered with aluminum duct tape, then the headliner. I have heard that Scamp
uses two layers of reflextix under the headliner. I have found that using a dry paint
roller to affix the headliner to the reflectix works better than using hands, as hands tend to stretch the ply able liner which results in wrinkles. Best of luck with your project. There is another type of marine headliner that is smooth vinyl, backed with foam rubber, so reflectix wouldn’t be needed as the foam is the insulating factor.
Dave and Paula