I have used a sheet of wax paper taped to the backing plate with good results. You can also put a couple coats of paste wax on surfaces you want to release. I also just finished a repair to a keel, where I spread thickened epoxy over the glass build-up, and then taped a piece of wax paper over the wet epoxy. I then used a plastic speader and a flat piece of wood to flatten and smooth the wax paper covered epoxy. When the epoxy had firmed up, I peeled the waxpaper, and had a smooth, flat surface that needed very little sanding and fairing.
Privateer 26 Schooner