I’ve built a lot of traditional style frame-and-panel cabinet doors with flat panels. The challenge is sanding the inside edges of the rails and stiles without scratching the panel, whether that panel is prefinished or not. My solution is to use scrap pieces of plastic laminate to protect the panel’s surface as I sand the inside frame edges. Slide the “protector” along as you sand the edges. In the past, I used thin cardboard as my protector, but it wears out quickly. Plastic laminate holds up to sanding much longer.