I found the best method is to sand all the parts smooth, can be painstaking doing it by hand but yields best results. An orbital will work but make sure you keep it moving so it doesn’t melt the part with the heat. Once sanded off move up in grit on the sandpaper just to minimise any large scratches left by the coarser grit. Once that’s done wash the part in warm soapy water and dry thoroughly. But a high quality plastic primer, of your part has minor imperfections get a high build primer so that you can build a nice layer and sand down any high sports to fix the imperfections. After a few coats of this, wet sand with 600. Clean and dry. Then a apply a high quality ‘texture’ paint. Gives the nice crinkly oem look. Few coats and done. You can leave like this if you like the Matt look. If not find a nice clear lawyer with uv protection and finish off.