To do this, you will need:
Sandpaper with 320, 800, 1000,1500, 2000, 3000 grit
You can get most of these components with a 3M headlight restoration kit available at any car parts store. Please note that this process can permanently remove the scratch-resistant coating of the iPhone which would make it more vulnerable for scratches in future. Also, this process can permanently remove the Apple logo on your casing.
Select a spot on your iPhone that has the maximum scratch and dry sand it with a sandpaper of the highest grit. If this does not remove the scratch mark, repeat with a sandpaper of a lesser grit. Continue the process with sandpapers of lesser grit till you are able to wipe off the scratch mark completely. Now, use this sandpaper to rub all along the surface of your iPhone casing. Remember to run along the surface in only one direction. Also, do not forget to cover your camera's glass lens to avoid scratching the glass lens.
Start with the sandpaper of the lowest grit and using very little water, rub over the surface in any particular direction. Once you have completed the process, repeat with a sandpaper of a higher grit but in a perpendicular direction. For instance, if you wet sand the casing with a 1000 grit sandpaper from left to right, follow it up with the 1500 grit sandpaper from top to bottom and so on.
The final step is to polish the casing. However, before you proceed, remember to cover up your headphone port, volume buttons, vibrate button, camera lens and 30 pin dock with a sticky tack. Once done, polish the casing with the 3M rubbing compound on a power drill. Once the entire surface is polished, you may finish the process with a 3M rubbing compound using a microfiber.
Optionally, you may also follow it up with a blue 3M pad with Meguair's 205 paint polish and finish it with Meguair's NXT 2.0 wax.
This should help in getting your iPhone 's shine back.
We hope you enjoyed this tutorial and let us know how it goes.