Wear on the Inside of Both Rear Wheel Wells

I've noticed that the factory black undercoating that's on the insides of both rear wheel wells on my Mustang are starting to wear out and disappear. I can see my car's paint color under the black undercoating on the inside of both rear wheel wells. Most of the wear is located on the inside left and inside right sides of each wheel well just to the right and to the left of the tire and on the top of the wheel well above the tire. How does the factory black undercoating wear out and disappear from the inside of both rear wheel wells? Does it wear out from the water splashing on to the inside of the wheel well while driving in the rain and snow? Is this a problem because Ford applied a very thin coating of black undercoating inside the rear wheel wells?
I am afraid that the exposed painted metal inside both rear wheel wells in the areas where the black undercoating wore off will cause the inside of both rear wheel wells to start to rust. Will both rear wheel wells start to rust because the black undercoating is worn off? Is there any way to fix this problem? I was thinking of purchasing some 3M rubber undercoating spray and applying it all over the inside of each rear wheel well. I am really afraid that the exposure to the salt that the bare painted metal inside both rear wheel wells already had by winter driving might be too late to prevent rust from appearing even if the inside of the wheel wells were sprayed with undercoating right now. The inside of each rear wheel well has already been exposed to the winter road salt for the last 4 years of winter driving so if the 3M rubber undercoating is applied to the inside of each rear wheel well to cover the exposed painted metal, will the metal rust after it is undercoated because it already went thru 4 years of winter driving in road salt?
I never noticed that there was any exposed painted metal inside both rear wheel wells until I happened to inspect my car up on a lift while the oil was being changed.

Is there anything to worry about once undercoating is applied to the inside both rear wheel wells to the exposed painted metal?

Do you think that the damage has already been done and that the inside of both rear wheel wells will start to rust because the factory undercoating wore off? Or is there nothing to worry about?
Please tell me what to do to save the exposed painted metal on the inside of both rear wheel wells from rusting.

Thanks