2004 Toyota Prius body rust / corrosion (rocker panels, front fenders)
 

NOTE: I split this discussion from my Prius build/ecomodding thread because it's something specific that other owners will be searching for more and more as these cars age.

---

My 2004 Prius is rusting out in several places.

It's worst in the rocker panels, but the front fenders have also started to go. This thread will show where and hopefully why the corrosion has started,so owners of newer cars (or older cars in less rust-prone parts of the world) can take steps to prevent it.

Eastern Ontario & west Quebec are perfect laboratories for discovering automotive design flaws that cause rust by trapping moisture, winter road salt (the killer) and dirt.

Here's what the rocker panels look like when I got the car

http://ecomodder.com/forum/attachmen...1&d=1437675683

http://ecomodder.com/forum/attachmen...1&d=1438795933

It was easy to poke that screwdriver through the side of the car.

I have an idea about the cause/path of this rust, and hope to offer some good preventive info to owners of newer cars (or cars that live in less salty winter environments) that haven't begun to rust... yet.

How do you think the rust started?
It looks like it started from inside. Rusted trough wheel well got dirt in there? Some clip from bottom plastic paneling fell off and allowed dirt in there? Just by itself? Some other way?

Maybe you (mostly meaning other Prius owners) can put rust preventing oil into there from rear spat attachment hole.

On the 3rd gen, that whole area is covered by a plastic rocker panel.

http://s3.amazonaws.com/images.holla.../5IG/SJL/2.jpg

I wonder if this was one reason Toyota made that change. Is rust in that area common?

It's definitely rusting from the inside out. The rust is a lot more widespread than the photo shows because once the paint has blistered, the metal beneath it is all corroded.

I haven't opened up the rocker panel yet to confirm, but I believe the problem has to do with this "shelf" at the bottom of the rear wheel arch (ahead of the rear tire) that holds dirt, salt & moisture:

http://ecomodder.com/forum/attachmen...1&d=1439047072

You can't see it well in the pic, but where the yellow arrow points, the tip of the screwdriver that I stabbed into the car is visible at a small hole there. Which means water, salt & dirt are getting in the rocker cavity there.

Also, all the factory undercoating has flaked off that shelf, exposing bare metal.

Exactly! I would highly recommend drilling the rocker panel where the rear doors close (above the door seal) to spray something for rust prevention inside the cavity.

And when I cut the existing rust out of the rocker panel to repair it later this summer or early fall, I should be able to confirm the source.

The problem with those plastic panels is if there's still a design flaw in the car, you won't know that the car is rusting out behind the plastic until it falls off! I've seen lots of cars with plastic rockers that hide serious corrosion.

Yup. Now that I have the Prius, I'm paying attention to them, and have seen the rust bubbles on the rockers of several earlier 2nd gen cars around here.

front fender rust too
 

I'll post a pic later, but there's also rust starting in the front fenders as well, behind the tire.

It's in the location of the holes in the wheel arch where the plastic inner fender liner & mud guards attach to the panel. On the passenger side of my car, the corrosion has crept around the corner from the tabs out onto the side of the fender.

UPDATE: some pics...

http://ecomodder.com/forum/attachmen...1&d=1439053569

http://ecomodder.com/forum/attachmen...1&d=1439053569

http://ecomodder.com/forum/attachmen...1&d=1439053569

http://ecomodder.com/forum/attachmen...1&d=1439053569

Maybe you don’t need to drill a hole. (From parts diagram) It looks like rear tire spat is attached to plastic plug with one screw. So maybe you can remove the tire spat and then pull the plastic peace of and then use its hole to put oil in there. Or maybe you could use the attachment holes for rear door sill panel.

Prius rocker panels have inner structure. Only pictures that I have found:
http://techno-fandom.org/~hobbit/pix/hf07/156aby-ch.jpg
http://priuschat.com/data/attachment...5_23.31.29.jpg

Looking forward for your pictures.

EDIT: On the other hand is it even that beneficial to protect the inside of the rocker? If that hole was rusted and worn into the wheel well wouldn’t just stopping the hole from appearing stop the rust from appearing? Or is the hole in there from the factory? Or did that hole start from inside?

Thanks for those picture links of the rocker structure! We'll have more when I cut out my rust.

I suspect the hole on the "shelf" in the wheel well was caused by rust, and then salty water & dirt got inside from there. So if you could prevent the hole, you prevent the rocker rust.

I would still rust-proof the inside of the rocker. It's normal procedure (commercial aftermarket) here to drill access holes to spray rust inhibitor inside hard to access areas. Then you put greased plastic plugs in the holes.

I'm posting pics of the front fender rust in a minute. I'll add them to the previous post, above.

Front fender rust might actually be caused by attaching the front mud guard. Those J-nuts will wear (or even rip while attaching) paint off.

We use lots off road salt here. But for some reason Priuses don’t have front mud guards (which off coarse causes the paint to wear from rocker and then it rusts from outside). But since front inner fender liner attaches with plastic clips there’s no rust in there.

My outer rockers are totally gone. I can see the ground behind the plastic panels. :)

There’s already a hole for rust protecting the inside of the rocker panels. If you remove the rear tire spat, underneath it there’s a hole for rust protecting. Tire spat has seal in it that will seal the opening. You also need to remove the rear tire to have good access to that hole. Spat is held on by one screw on the bottom of the car. Screw screws into plastic so it opens easily.

From this hole you can push in the rust protecting spay can tube that has multi nozzle at the end. http://www.tectyl.fi/2009/tectyl_ml.jpg this is the stuff that I used. Condensation on outside of the panel clearly showed that all of that area (and more) that was rusted in MetroMPG:s Prius was protected.

Often caused by blocked drain holes at the bottom of the engine/pax compartment firewall where rotting leaves, dirt and other crud combine to form an acidic mess that eats cars alive ...

Often overlooked during maintenance until too late.

That was my next question: whether that hole provided access to the vulnerable/affected "compartment" inside the rocker structure.

I was going to wait until the fall to do this repair, but my curiosity has got the better of me: I think I'll cut open the rusted part of the rocker panel tonight. I'll post pics tonight or tomorrow.

Confirmed the source of the rust is the hole in the corner of the "shelf" in the wheel well (see the photo from the wheel well side with yellow arrow pointing at hole in a previous post):

http://ecomodder.com/forum/attachmen...1&d=1439248845

Those rust flakes are sitting loosely on the bottom of the rocker panel, I knocked them off the inside/side surface when I was stabbing/probing/banging that area.

Below is the view looking forward -- you can see the rust happily spreading out inside the cavity. Also: evidence of the elusive Canadian Rust Spider, which catches tiny rust flakes in its web! :D Mmmm, iron oxide.

http://ecomodder.com/forum/attachmen...1&d=1439248845

Discovered possibly easier ways to access inside the rocker panel:

http://ecomodder.com/forum/attachmen...1&d=1439248845

1) This is the screw you described for removing the rear tire spat/deflector. The screw threads inside my rocker were so rusted, I didn't even bother trying. It's one with the car now.

Multiple, easier-to-access points inside the rocker run all along its length in the form of the fasterners that hold the plastic aero panel to the underside of the rocker itself.

2) There's one round plug/fastener (already removed, see below).

3) The majority of the plugs/fasteners have rectangular bases that make up part of the smooth surface of the panel.

Below: Peek-a-boo through #2.

Also, to the right of the plug/fastener hole, you can see the dirt and grit that has gotten into the rocker through the rust hole in the "shelf".

http://ecomodder.com/forum/attachmen...1&d=1439248845

The plugs/fasteners popped out easily for me by prying with a regular flat screwdriver:

http://ecomodder.com/forum/attachmen...1&d=1439248845

http://ecomodder.com/forum/attachmen...1&d=1439248845

I would highly recommend that any gen. 2 Prius owner open the forward & rear plugs/fasteners under the rocker panels and spray a liberal amount of rust inhibitor inside. Both sides of the car.

And if you have any rust at all on the shelf area, clean it up, re-coat the area with a suitable sealant (normal paint won't stand up to the road grit thrown at this area) and do regular rust protection via the plug/fastener holes.

So glad I don't live in those conditions. I remember an early 70s Buick that came from upstate New York that had a rust hole in the fender 6 inches wide. The car was 6 months old based on the production date on the vin tag.
Vegas were pre rusted with no primer under the (painted) headlamp bezels.

regards
mech

Great info. I'll have to check my car out.

Yeah, Vegas even rusted in Southern California :-(

And I remember a friend's Mustang, which was interesting to drive in 'cause you could see the road going by under your feet. Couldn't have been more than 5 years old at the most...

But think of the fun you're losing out on -- picking out a new winter beater each year!

---

I had another thought: if I were planning to keep the car, I would also pull all the plastic undertrays off to inspect the floor. They're potential rust traps.

On this episode of Prius Rust CSI...
 

Yesterday evening I opened up the passenger side rocker panel.

It's not quite as rusty as the driver's side. Interestingly, there is no rust hole on the "shelf" inside the wheel well (though the coating is gone from it, and the bare metal is rusty).

So how is salty water & dirt getting inside to make a mess of things?

I think Speeed3 already explained it:

Here's the tire spat/deflector.... showing telltale signs of rust beneath it:

http://ecomodder.com/forum/attachmen...1&d=1439483422

Spat removed, you can see the factory drain hole in the end of the rocker (which I already cleaned out before I took this pic):

http://ecomodder.com/forum/attachmen...1&d=1439483409

Inside my rusty rocker was a lot of very fine grain grit & sand. Most of it was concentrated near that hole. How did it get in?

Toyota put spongy gasket material on the spat where it contacts the wheel arch in an effort to prevent grit & water from getting inside the drain. Gasket highlighted in this pic:

http://ecomodder.com/forum/attachmen...1&d=1439483409

But judging by the amount of fine grit and rust I found in the rocker in & around that drain hole, it's clearly not an effective seal.

Another view of the drain from other side of the crime scene:

http://ecomodder.com/forum/attachmen...1&d=1439483409

Moral of the story for gen. 2 Prius owners: pull those plugs/fasteners out of the bottom of your rockers and get some rust-inhibitor inside.

Reason why I chose to use the opening behind tire spat/deflector is that I feared that opening those plugs might get them lose. If the plastic plugs(s) from the bottom of the rocker fall off you can’t really even see it and it will allow lots of dirt inside.

That’s why I would recommend using the access behind the tire spat/deflector for rust protecting.

That screw is treaded into plastic peace so no matter how rusty it is it should still be removable.

The 2 large plugs I removed went back in tightly, but I can see your point.

If anyone takes the vehicle for a professional spraying, I would definitely show them the larger plugs so they can get their (large) wand inside the cavity. Typically they only drill access holes on top of the rocker (inboard of the door seal), and in the Prius' case, that means they'll only be rust-proofing the inner rocker compartment, not the outer one with the rust problems.

If you planned on keeping the car, would it do any good to cut foam to fit inside, so that it does not fill with debris?

If you had a car made of all fiberglass...it would pretty much last forever.

In my experience, it is not just the holes that are causing the rust, it is much more the fault of condensation and poorly protected metal on the inside of the rocker. The poor coating is added before the panel is welded in place, so the spot welds become areas for immediate rust.

If you get snow caked on the rocker, and warmer air inside, you'll get water beads forming inside = rust.

Solution: as soon as you get a car, use the existing holes and adding holes if necessary to spray in oil based substance to coat the interior. I use CRC brand white lithium spray grease, WD-40, or similar with the tiny red spray straw to spray inside the rockers and fenders.

A totally easy spot to reach is on top of the wheel arches thru the trunk area, just pull back the liner or remove the plastic and spray on top, or use a brush to apply grease.

Again, I really think its the condensation that occurs, not the few holes here and there in the rockers.

The amount of debris -- sand & fine grit -- piled inside the rockers in the areas by the rear drains where it's rustiest would challenge the idea that condensation is the main culprit!

If grit is getting inside, then so is salty water in the winter, which is much worse than pure condensation.

But condensation would absolutely make things worse, especially where rust has already started.

Regardless, spraying rust inhibitor from day one, like you do, is best.

rocker repairs begin
 

I drove the car through the winter with heavy duty black duct tape over the holes where I had cut the rusty sections out of each rocker.

Now it's coming up to time to sell the Prius, so the repairs have begun!

Before:

http://ecomodder.com/forum/attachmen...1&d=1439248845

After:

http://ecomodder.com/forum/attachmen...1&d=1465499318

Above: shows a metal patch butt-welded into the opening I had cut. I'm no pro welder, and the metal was perty thin in a few places, so I blew a lot of little holes while welding. I epoxied over the holes for a waterproof seal.

Next up: body filler & smoothing.

Looks like a good start.

Skim coat of body filler:

(Yes, I'm this slow.)

http://ecomodder.com/forum/attachmen...1&d=1466040121

Wow that is so low, did it even really need a skim coat?

Oh, my metal patchwork isn't as smooth/flush as the photo might have lead you to believe. It does need a little filling/leveling.

Also, I ground down a few other spots where the rubberized undercoating was starting to bubble from underneath, and you can't feather-sand the edges of that stuff. Plus, it's pretty thick, so needs a bit of fill to make it all level.

Rockers done
 

And I'm happy with the outcome:

http://ecomodder.com/forum/attachmen...1&d=1466430504

Above: passenger side body filler smoothed, rocker sanded for painting.

http://ecomodder.com/forum/attachmen...1&d=1466430504

Rubberized rocker coating.

http://ecomodder.com/forum/attachmen...1&d=1466430504

Painted - one nice thing about a black car: easy to get a colour match.

http://ecomodder.com/forum/attachmen...1&d=1466430504

The only obvious sign of the repair is the tape line on the dogleg where the rubberized layer stops. The OEM coating ended lower down and was blended instead of cut off. But if you didn't know the car, you might not notice.

I'll spray some more rust inhibitor inside the rockers for good measure, re-undercoat in the wheel wells and that will do.