When a gun is dipped (camo) properly the final step is usually a PPG automotive grade Matte clear coat. You can get this at any automotive paint supplier or take the gun to your local body shop and have them shoot it for you. you would have to break it down, Remove all loose parts and degrease it or have them degrease it for you. then they can shoot it for you and have it back the next day.
But why do you need this? the finish on a Benelli is very good. My Black (HK version) never rusted even when accidently left in a wet case.
Hope this helps.
Let us know what you do and how it works out.