I was watching a YouTube video last night and the guy was doing some troubleshooting on a car but not a Prius. He had to remove some of the plastic panels from under the car to troubleshoot a wiring problem. You would be surprised at the amount of dirt the panel held, he swept it up into a pile and it was six inches deep. I always though those panels would help protect the car somewhat from rust but not if they fill up and stay wet all the time. I have never removed any except the very front ones from my old 2008 Prius band the new one is still a virgin. What have you seen regarding this? The car I am talking about traveled a lot on dirt roads in upstate New York.
When I changed the oil on my 2010 I always removed the "engine under cover." There was always a lot of debris (rocks, dirt) collected there. Not surprised it's the same for the Gen IV.
The bumpkin in the thread did a front wheel bearing hub assembly replacement on a 2012 Prius I watched. He just kept the impact gun on the locking nut for 2 solid minutes. Let’s just say if you were to do that, the threads on the axle would be so rounded due to the dimple placed in the locking nut that thread repair is required . But plenty of dirt to go around from that video though.
I've had a few sets of covers off previous cars - a '95 Mazda was full of small leaves, and an '90 Mazda which spent it's first 15 years in dusty Western Qld had dried mud caked under there - which explained the rust bubbles. My last 3 cars I didn't look as I intended to sell them about when the warranty expired.
We’ve driven our 2016 Prius 2 on gravel roads in Minnesota for 2 years, and just noticed that the plastic panels under the car in the driver/front passenger areas are full of gravel and sagging 3 inches from the added weight. Cleaning them out leaves them permanent deformed and likely to fill up all the faster. Replacing them will cost $540 plus the cost of the labor. This is a major design flaw!!!
I would look into something home brewed to make a decent repair, without looking at it I can offer no further help except to not put it off.
You have to periodically raise the car and have at it with compressed air I guess. Or a leaf blower, wet/dry vacuum on blow, what have you. Goofy to need to do this, the price for underbody aerodynamics. On our 3rd gen if I tap the long mid-body underpanels, stuff always vibrates out.