Marlite wall panels are made from a fiberglass-reinforced plastic and typically are glued straight onto a wall's sheetrock, either as a whole panel or as smaller tile-sized bits. These panels usually are used to simulate a specific look -- marble or slate, for instance -- without actually being made of the more expensive materials. These panels are glued onto the wall, so it often is easier to just paint over them rather than try and remove them and then paint.
Mix a bucket of warm water with dish-washing soap. Use a sponge to clean off the Marlite panels. Remove all visible dirt and grime, as even a small bit can cause a blemish in the paint job.
Rinse off the panel to remove any remaining solution, then dry the panel off with a towel or rag.
Lightly sand down the surface of the Marlite panels. Do not over-sand; make a slightly rough surface so the primer and paint establish a stronger bond with the fiberglass.
Dampen a towel or rag and run it over the surface of the Marlite panels to pick up any dust or particles that occurred as a result of the sanding.
Apply a coat of primer to the Marlite panels and allow it to dry for the length of time specified on the primer can.
Mix the paint according to the directions on the paint can, then use a paintbrush or paint roller to apply the first coat of paint over the primer. Use smooth, up-and-down strokes; do not worry if some of the primer or Marlite color shows through the paint. It often takes three or four coats of paint to achieve uniform color and texture.
Wait for the first coat of paint to dry, then apply more coats as necessary.