My 2016 Fit has a place on a rear wheel well that has rusted through. The warranty covers rust-through for 5 years. Mine is 6 years old.

I've always kept it as clean as possible. Grrrr. I put rust treatment paint on the spot. Don't know what else to do. Fixing it seems pretty involved and I'd bet its expensive to have a body shop fix it.

This sucks.

 

Sorry to hear about the rust. Pictures will allow others to give better advice.

Based on your description (e.g., "rusted through") a proper repair will likely be somewhat involved and costly.

If you want to stop rust you have to remove all corrosion and fix whatever caused the damage in the first place. In the case of a badly rusted/perforated panel, this will involve cutting away all rust back to clean/undamaged metal and fabricating a replacement, followed by properly applied sealant/primer/basecoat/clearcoat. For most of us, this is a job for a body shop. To give you a very rough idea of cost, we bought a 2014 Nissan Leaf with a rusty rear wheel well lip (about 1 x 4 inches of corrosion and paint bubbling). Our local (central MA) body shop cut out the bad spot, rebuilt the fender lip with new metal, and repainted the entire rear quarter panel for approximately $1250. The repaired fender lip is indistinguishable from the other side. That's a decent chunk of money, but I got a very good price on the car so the added investment was worth it for us.

In the case of surface rust, you may be able to get away with grinding away rust to bare/clean metal, filling in the depressions with body filler, and then painting properly. Depending on the size and location of the repair, this can be a DIY repair if you have the time and energy to watch a bunch of Youtube videos and don't cut any corners with your work. If you get lucky and do a good job, this can be a permanent repair assuming you have fixed the underlying cause of the rust.

I'm not sure exactly where your damage is, but there are definitely locations around the rear wheel (of many cars) that are traps for dirt and moisture (and salt if you face that evil). Washing the car regularly can help, but if you don't specifically target the "traps" you probably aren't removing the muck that builds up there.

Rust treatment paint (I assume you're talking about the various products that purport to convert rust to an inert form and seal it) can slow the spread of rust, but if you don't fix the underlying issue that caused the rust you are fighting a losing battle. If you take this route, your best bet is to keep the rusty area as clean and dry as possible. If bubbles or flaky rust show up you need to get rid of them as they trap more moisture and accelerate further corrosion.

Good luck.

 

Here's a picture. I scraped the area down to metal then painted it with rust-treatment paint. It's on the front section of the left-rear wheel well.

 

That is an interesting location, as it's not what I would call a "trap" in the manner I described in my previous post.
I believe there are two panels in this location (outer and inner) that are bonded along the bent lip of the fender. When you said "rust through," did you mean that there was a hole completely through the outer body panel or both panels so that you can see through to the inside of the wheel well? I think you're talking about the former, with small holes through the outer body panel, correct?

I'm asking so that we can better speculate on what caused the rust in this location and how you might fix that underlying issue. If there is only damage to the outer panel, the most likely cause would be some sort of damage that compromised the paint. Perhaps an impact/scratch to bare metal. Did you notice this in the past? If this sort of damage is the cause, you might be looking at an easier fix (depending on how nice you want the repair to look) with a limited chance of further rust.

Alternatively, if moisture/dirt/salt can get in between the inner and outer panels without a way to exit they will eventually rust from the inside out, although in that case you would expect the rust to first appear lower on the panel. It would be worth your time to carefully check the condition of the metal lower on the panel below the rust. Any bubbles? Any difference in sound if you tap with your fingers? Does your finger poke straight through the panel? (Not a joke, I've seen a body panel rust from the inside until all that was remaining was a bubbly layer of paint.) How does the same spot look on the other side of the car?

Is there any separation of the two panels along the lip of the fender? Any other damage inside the wheel well (e.g., check for cracks in the undercoating and seam sealer between panels) that could allow moisture or dirt to get in between the panels? If there are any rubber underbody plugs lower on this panel it would be worth popping them out to see what is inside: moisture, dirt and other gunk, rust chunks from degrading body panels. If that's the problem, it's going to be more difficult and expensive to fix if that's what you decide to do.

 

I will check further but from what I can tell I think the rust only penetrates the outer panel - like you described in the second paragraph.
It seems I remember hitting a branch or something similar that came up and hit the car.
I will be changing oil in the next day or two. (Been waiting for the temps to fall below mid-90's.) I'll give it a better look then. Maybe even remove that tire to get a better look at the backside.
I'd be happy just to fix it to stop the rust even if it leaves a scar.

 

Pretty sure it just penetrates the outer panel. Changed oil yesterday and did some more looking. Any ideas how to treat this, other than just keep on applying the rust treatment paint? I took some more pics with my endoscope but I don't think they really show much more.

 

It's hard to say much more from the pictures we've seen.

Your best bet at this stage is to visit a quality body shop. Most can give you a quick estimate and explain what they would do to fix the damage. Maybe it's reasonably priced and you're happy to go with that, or maybe it's way more than you want to spend. As I indicated in a previous post, even a small repair can get pricey if fixed properly.

If you want a home fix, and can confidently determine that the rust started on the outer surface due to external damage, you can probably do a decent DIY repair: basically grind away the rust, treat the bare metal, patch any holes and fill in the low spots with body filler, and then primer/paint/clearcoat. The quality and appearance of your repair will depend on the time and effort you take.

If the rust came from the inside out, it's likely worse than what you can see externally and also more difficult to fix or stop as a DIY repair. Not impossible but much more involved.

At a minimal level you can try to freeze the rust at the current stage by keeping the area as clean and dry as possible (rust requires moisture and oxygen). You can regularly apply converter paint, or perhaps try a product like PB Blaster Surface Protect or Woolwax. I have used the former on the underbody of my wife's Leaf, which is daily driven in Massachusetts winter road conditions. I put it on around Thanksgiving and it was still there in the spring, so it's at least providing a long-lasting coating. It's sticky so it won't look great on the fender of the car, but neither does rust or rust holes.

You have to weigh your available time and budget vs the value of the car (both monetarily and to you) to evaluate what approach is best for you.

 

You clearly live in the rust-belt. All vehicles rot out due to the materials they use to removing snow and ice from the roads. Most people drive cars for a few years and then buy new ones. No metal longevity like living out west!

 

Thanks. I think I'm just going to live with it and use the products you mentioned. They put so much salt on the roads around here it piles up in the intersections.