Sanding plastic, especially plastic car body parts, is essential if you need to get a perfectly smooth finish. The primary thing to remember when wet sanding plastic is to use a light touch. Let the paper do the work. Never sand in just one direction as this will result in scratches that are difficult to cover later. Instead, sand in a circular motion, using circles of differing sizes so there is no discernible pattern to your sanding. Use a circular foam sanding pad that accepts adhesive-backed sandpaper. A nice foam pad allows you to sand both flat surfaces–as well as surfaces with a curve to them. Start with a 200-grit paper and progress to an 800-grit paper for a perfectly smooth finish every time.
Wash the plastic that you will be sanding. Use a de-greaser if necessary to remove all dirt, grime and grease. The cleaner your surface, the better.
Fill a bucket with water. Alternatively, fill a spray bottle with water.
Cut a piece of 200-grit wet/dry adhesive sandpaper to fit a circular foam sanding pad. Adhere the paper to the pad. Dip the pad in your bucket of water, or spray both the paper and the plastic surface you are sanding with the spray bottle.
Sand very lightly using circular motions. Vary the size of your circles so you are not going over the same path over and over with your paper. Your sanding should be as random as you can make it. Frequently, re-wet the paper and the plastic surface. Clean plastic build-up from the paper occasionally.
Switch to 400-grit paper once you have lightly sanded the entire surface with your 200-grit paper. Again, keep the paper and the plastic surface wet, use an extremely light touch and sand in random circular patterns.
Wipe the entire surface with a damp rag to remove any dust or sanding debris. Inspect the surface carefully for spots that are not fully sanded. Rub your fingers over the surface, feeling for any rough spots. The entire surface should feel as smooth as glass.
Place a piece of 800-grit paper on your foam sanding pad, dip in water and make one last pass over your plastic. Use almost no pressure. Keep the paper and the plastic surface wet. Sand in random circles that constantly overlap each other so there is no discernible pattern to your sanding. Wash and pat dry. Your plastic is now sanded as smooth as glass.