Repairing rust on rear differential

[Pyral]

My questions are:
How should I deal with rust that is in sensitive areas like the area right next to where the drive shaft goes into the differential? Should I try sanding and painting them best I can or simply paint over the rust?

What is the best method of removing rust from the textured cast finish that you get on the differential housing? I've already tried sanding.

What should I use to prime (for painting) in areas where I have a mixture of bare metal and rust I might not get to?

Background:
My 4runner was painted before I bought it, and for the most part that paint has worked well. But today I learned it didn't work well on the differential housing.

I was repairing a small rust spot on the differential and as I was repositioning the painters tape the paint lifted up with the tape revealing more extensive rust. This happened multiple times around the differential.

My biggest problem is how to approach removing the rust, I normally use sand paper but the differential housing is a cast part with a texture that doesn't allow me to fully remove rust from the textures. I tried a Dremel wire brush and that didn't quite get everything, and the complex shape made it hard to use in many areas.

The next problem is the rust runs right of the the edges of important features. For example it goes right up to the edge of the drive shaft, around some of the bolts that hold the diff together, where the diff housings join, and the drain plug. Sanding rust away while avoiding the sensitive areas is bad enough, let alone spray painting it. I'm not sure how or if I should deal with rust here.

The last problem is what primer I should use. If I can't ensure that the rust has been fully removed, then I wont get proper adherence of primer, and I'll be in the same position a year from now when. The primer I used in one area today was a metal primer, and I'm not happy because the primer was very runny and I'm not confident it will work on parts that might still have specs of rust.

 

[brillo_76]

The 3rd member and front diff oudomg are cast iron. You use screw drivers and flack the rust off. Once you get it all off degrease it and just paint it.

I would not use corroseal under por 15 or kbs seal on top. It will peel of in a year or 2.

Now metal prep and por 15/ kbs seal might be ok.

I just clean it and just throw paint on it. Once dry I rust inhibitor solutions on top.

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[Pyral]

Got some photos.

Here's a surface that I've already sanded and extensively taken a wire brush to. Despite that it still has spots that look brown. I'm not sure if I should paint over that or keep trying to remove it. The grey is bare metal, the black is paint that I haven't sanded. The dark brown might be rust, I'm not sure.

The photo also shows the challenge I'm facing. The rust continues into the gap between the diff housing and the driveshaft, and I'm not sure how to handle it.

Heres another problem area. Rust is right at the diff seal, I don't want to mess around there but I also don't want to leave it like than. You can also see this dark brown color. Typically the rust starts off as a light brown or orange, after sanding it it becomes this color. I'm not sure why the color is so different, but it doesn't take a lot of sanding to get rust to get there, but to remove it completely it takes a ton of effort.


Here's more photos. A lot of the paint was picked away by me, after realizing that these rust spots weren't contained I took a metal needle and would pick at the paint to see if it was compromised. My differential doesn't look bad, but I like to fix it while the problem is still only a cosmetic issue.

 

[Speedy]

Wire wheel the loose stuff, spray with Rustoleum Rust Reformer, top coat with Rustoleum satin black.

 

[Sean K.]

Wire wheel the loose stuff, spray with Rustoleum Rust Reformer, top coat with Rustoleum satin black.

^This. Wire wheel with a 4.5" angle grinder and 4-4.5" wire wheel.....easy peezy and you can use that grinder for all kinds of stuff. I personally like paddle switch Makitas....but that's just me.

 

[brillo_76]

Cast iron is interesting . It's harder to keep from rusting. I just clean it and paint.

Paint is just an indicator anyhow.

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[Speedy]

[MENTION=164664]brillo_76[/MENTION] is the resident rust expert :beer:

 

[brillo_76]

Not a rust expert. Just always looking for better methods...

Nothing wrong with wire wheels...

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[Speedy]

Well, except when the little wires fling out and stick in your hands and arms. Be sure to wear safety glasses.

 

[brillo_76]

True. Everything needs safety glasses.

Dremel would just burn through materials but in super tight spaces very handy.

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[brillo_76]

OP how are you making out on this project?

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[diverscale]

I air blow then spray grease based anti rust all over it during my annual anti-rust, and sometimes another shot during the spring at oil change. Painting without sandblasting these is worthless. You'd be better off wire-wheeling it to remove scale and periodically anti-rust. That would also take care of that seam

 

[brillo_76]

I air blow then spray grease based anti rust all over it during my annual anti-rust, and sometimes another shot during the spring at oil change. Painting without sandblasting these is worthless. You'd be better off wire-wheeling it to remove scale and periodically anti-rust. That would also take care of that seam

You need to acid etch the steel to get the paint to adhere to the surface of the steel.

Nothing wrong with oil based anti rust solution. Helps keep the oxygen away from the steel surface.

I seen steel rust undercoating if not new metal.

I am currently wondering if I am observing cast iron rust under leaking axle seal on my sienna van rear diff. Not sure on this one as I am still investigating that one.

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[Devbot]

Sand, wire wheel as much as possible, apply rust primer and paint as needed. If you really want, pick up a cheap portable sandblaster and some media to help you reach the tough spots (money very well spent, believe me.) I wouldn't lose sleep over a little bit in the cracks here and there, just address it whenever is convenient and I think you'll be fine. I wouldn't go splitting the third member apart just for that little external corrosion
Does Eastern WA use salt? I know Olympia (/Seattle) are pretty against it, but I've also heard some smaller towns/localities use brine here and there. And you guys have much harsher winters than us to boot

 

[shadow247]

I can tell you not to bother with the Harbor Freight Needle Scaler. The needles are softer than the cast iron, so it basically did nothing. I ended up getting in the smaller cracks with an assortment of wire wheels from harbor freight.

 

[Devbot]

I can tell you not to bother with the Harbor Freight Needle Scaler. The needles are softer than the cast iron, so it basically did nothing. I ended up getting in the smaller cracks with an assortment of wire wheels from harbor freight.

Do NOT use wire wheels from HF. Unless you like surgery... Pic is me last year, went clear to the bone.
Spend the couple extra bucks on a decent brand. Jaz, Weiler, etc.

 

[brillo_76]

Do NOT use wire wheels from HF. Unless you like surgery... Pic is me last year, went clear to the bone.

Spend the couple extra bucks on a decent brand. Jaz, Weiler, etc.

Don't use their cut off wheels either. Best to buy better quality products. I had those shatter...

Bummer in the wire wheel incident..

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[aontkos]

Ooof, a burning hot strand of metal covered in rust dust flying into your arm meat sounds pretty unpleasant...

 

[Speedy]

Do NOT use wire wheels from HF. Unless you like surgery... Pic is me last year, went clear to the bone.
Spend the couple extra bucks on a decent brand. Jaz, Weiler, etc.

No wonder I had so many shards flying at me. Mine were HF. Guess I'll throw the rest in the trash.

 

[Pyral]

I'm not using a proper wire wheel, it's simply one of those attachments that you can buy for a Dremel. Any time you use a power tool you should wear safety glasses and gloves which I am.

Every time I pull up the painters tape after I'm done with one area it peals up more paint right next to it. It's a frustrating cycle, if it hadn't kept pealing I would of been done 3 days ago.

I notice that it never seems to have any primer, so that might explain the poor adherence. Use primer folks, it's there for a reason. I'm using this stuff, hopefully it'll be affective.