When you look for rust, look for spots where rust starts easily. I was washing the undersides of my fenders and I found dried mud in the lip. It is 'U' shaped and well designed to catch all the mud. Even after a good soaking the mud stayed there. It was dry and it was hard to make it wet enough to wash out. The best time to wash it out is after a few days of driving in the rain. The rear lip has underseal in it but not enough. The front lip has the fender liner to limit the gap, but at the top there is a perfect pocket to catch the mud. Keep that mud damp and the rust will start and when it starts it never rests. Add a little salt and it is accelerated. I think I will fill these gaps flush with black silicone after a good cleaning. It will stay flexible and hopefully stick to the underseal, if it goes behind the silcone it will be worse than not having it. No mud - no rust.
Hose out all the pockets where silt can get thrown by the wheel spray, on some cars you are amazed at the weight of mud you can carry without knowing about it.
I tried to clean the fender liner but it was too much like hard work, but some bug and tar spray got on it by accident, wiped it of and it was liken new, problem solved.