I am not sure how helpful paint protective or any film will be from a hard impact on a mountain bike trail. However, for things like rocks being kicked up or a light scrap, I think paint protective film will work well.
I used XPEL Track Wrap on my Road Bike and it works well to protect the paint from rock chips and cable rub. It is easy to apply (i.e. no soap sprays like other films, etc.) by just pressing it to the surface and it peels off without residue easily if you want without the need for a heat gun or hairdryer. Also, it is pretty cheap vs. other options if you are willing to cut it to size yourself.
I have never had issues with it pealing off and have had it on my bike for over 1.5 years and several thousand miles. Also as a bonus tip, I use it on my car with my trunk mount bike rack to prevent the feet from marring the paint. So far that has worked well too.
I put sections of the film on the following areas:
- Top Tube
- Under Bottom Tube to Protect Against rocks from the front wheel
- Where cables rub the front of the bike before they enter the internal grommets on the frame. It definitely prevents marring of the paint there. I forgot to do it on the rear brake cable under my seat and see the difference.
- You could possible use this too on the chainstay or use something like Lizard Skin which I now use after some chain suck smacked my chain into the stay.
You can see from the pic below it is clear enough that it cannot be noticed unless you get inches away from the frame. I also use a cutting board to cut long straight sections of the wrap in straight lines to make applying it easier and the results better looking.