Polishing is the process of sanding your part smooth and then bringing it to a glossy, mirror-like finish. Higher quality finishes will require exponentially more time. Test your polishing process on scrap carbon before you begin on your real parts.
Ensure that your clear coat has fully cured and that the surface is free of pinholes. If there are pinholes, use a fine tip brush to carefully dab a small amount of clear coat into the holes. After the dabs have cured, fill a small dish with water and a few drops of dish detergent. Allow your sandpaper to soak in the dish for a few minutes before use. The exact process will depend on the hardness of your surface material and the desired finish.
Begin by using a grittier sandpaper (800-1500) to spot-treat any small imperfections, sagging clear coat, or runs. Remember to sand with a forward and back motion, never in circles. Straight lines can be sanded out with perpendicular blocking, whereas circular sanding marks are much harder to remove. Step up to a finer grit (1500-2000) to remove overall texture from the part and leave a flat finish, and if needed finish with an ultra-fine grit (2000-3000). Fully dry your part and examine it for remaining surface imperfections.
It is imperative that no particles or debris get under the sandpaper, as they will cause deep scratching in the clear coat. You should regularly rinse your sandpaper and part to clear material build. Listen and feel for debris under the paper, stop sanding immediately if you detect any. You can either hand sand or use a double-action (DA) rotary tool for the wet-sanding step. Use light pressure, especially when sanding with power tools.
While polishing, take care to avoid over-heating the surface. Periodically check the temperature of the part with your hand. Dry the surface completely and spread M100 cutting compound across the surface. One or two quarter-sized blobs per sq. ft is a good amount to start with. Using a wool pad and firm pressure, work the compound into the part across the surface. Ideally this is done as a single, continuous pass to avoid polishing any one spot for more than a moment. Wipe clear with a microfiber cloth and examine for scratches. A small piece of 1500-2000g sandpaper can be used to carefully spot-treat any imperfections.
Apply more M100 and finish using a stiff foam pad. Wipe excess compound with a microfiber cloth and then use a second clean microfiber cloth to buff to a shine. The back of the part may be rather messy. Use a wet rag to clean it, or mask off your part and spray a layer of black paint across the back.