Any reason not to paint rear diffs?

I've noticed that in both my NA miatas, and also the stock diff on my ZHP all the diffs have quite a bit of surface rust. I guess they were never painted from the factory. Is there any reasoning to that besides saving a little bit of costs? Are there any downsides to cleaning the rust and painting it with POR15 or something?

I see no reason not to do it.  

^I concur.  They do get a bit warm under real work, so make sure to use a suitable temp paint.  I'm not sure off hand what temp POR15 is good for, but a solid chassis or engine paint will be more than sufficient.

The only reason I could possibly think of is heating issues, but I doubt paint would make enough of a difference to be an issue. 

If there's no reason not to, I just don't see why they weren't painted from the factory other than to cheap out.

Paint it. I usually end up with live axles so they aren't tucked up there like IRS but its still fun.

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A lot of factory differentials were painted, but in a cheap way (no primer, or paint that couldn't take the heat and grime) and the paint doesn't last. Using a good paint and applying it correctly won't be a problem.

Because they lasted 20+ years without failure due to it?

There's a difference between cheaping out and selling an inferior product.  Both of those cars mentioned have exceeded the service life by at least 2x, and they were both designed to be "affordable" sports cars..  There's a line to be drawn somewhere.  Sometimes you get the line right, sometimes you don't.  How else do you keep costs down enough to be affordable?

They painted the subframes of my Miata, but there's rust poking through.  They probably figured that if it was a cast piece that was an inch thick, the rust would take forever to wear through, no sense in protecting it (like chevy did with a bunch of suspension components in the truck...).

I always paint them, but then again, I like to detail my projects :-)

It can't hurt it, most are painted from the factory by most manufacturers.

Just not bright colors, if it is visible from behind.

akylekoz said:

Just not bright colors, if it is visible from behind.

I used to paint my diff covers in bright colors just for the "WTF" factor for folks behind me. I'm an odd fellow.

Painting a differential a bright color is a lot like a tramp stamp for your car. Whether you want that or not is an entirely different conversation.

slefain said:

akylekoz said:

Just not bright colors, if it is visible from behind.

I used to paint my diff covers in bright colors just for the "WTF" factor for folks behind me. I'm an odd fellow.

Depends on the situation, I'd definitely consider it. But then, all the Honda guys have their anodized rear suspension or subframe or whatever in the back to look cool. 

Thanks guys, I just had to ask. Unfortunately this particular scenario I don't have time to clean it up and paint, but there will be projects in the future, so this is good to know.

akylekoz said:

Just not bright colors, if it is visible from behind.

Strongly disagree, I think brightly colored parts on the underside of a car look really cool

The tramp stamp analogy isn't far off, I admit. More like a tattoo hidden in a fun place you don't normally see  Guess I have different taste in cars than women.

oldopelguy said:

Painting a differential a bright color is a lot like a tramp stamp for your car. Whether you want that or not is an entirely different conversation.

This.  When I replaced the rear subframe on my Miata it was a bit rusty so I cleaned it reasonably well with a wire brush and spray painted it bright red.  I figured her name was Brittney, she should have cute "drawers".  People liked it.

As far as a diff goes, go ahead and paint it.  I'm of two minds with a lot of suspension stuff, though.  I have seen many hundreds of examples where something that was heavily painted from the factory end up rusting dangerously, when the paint gets chipped, moisture gets under the paint, and you wind up with it being worse than just an overall coat of surface rust.

240 Volvo strut spring seats  were a great moneymaker for me.  It was fun to lift the car, smack the seat with a hammer and watch it explode.

I painted the 8.8 when i swapped it into my zephyr. Ive never seen a good reason not too. Debated painting the diff cover a diffrent color but decided to go all black because grandma car.

AWSX1686 said:

I've noticed that in both my NA miatas, and also the stock diff on my ZHP all the diffs have quite a bit of surface rust. I guess they were never painted from the factory. Is there any reasoning to that besides saving a little bit of costs? Are there any downsides to cleaning the rust and painting it with POR15 or something?

ZHP diff was originally painted black and had some factory paint marks.  Mine was getting a little scaly.  I painted it with POR15 on a foam brush and it came out looking great with minimal effort or prep.  But I'm weird -- I'd rather have my suspension, brakes, and engine bay clean and detailed than the top of the car.

Race car paint it white so you can see leaks when they are small and hard to find. Same with the transmission and oil pan. At least that is what I do. 

Street car, rattle bomb it black and call it a dfay. 

GameboyRMH said:

akylekoz said:

Just not bright colors, if it is visible from behind.

Strongly disagree, I think brightly colored parts on the underside of a car look really cool

The tramp stamp analogy isn't far off, I admit. More like a tattoo hidden in a fun place you don't normally see  Guess I have different taste in cars than women.

I love the look of bright colored, high contrasting undercarriage/suspension. Not so much the type of attention it gets. 

Then again, I did just buy a gallon of orange paint, for (what will be) my white car. So I guess it's not really that big of a deal for me. 

As far as tramp stamps, I have several friends that have featured on Suicide Girls, can't say that bothers me either. 

slefain said:

akylekoz said:

Just not bright colors, if it is visible from behind.

I used to paint my diff covers in bright colors just for the "WTF" factor for folks behind me. I'm an odd fellow.

You are not alone brother. 

Trackmouse said:

slefain said:

akylekoz said:

Just not bright colors, if it is visible from behind.

I used to paint my diff covers in bright colors just for the "WTF" factor for folks behind me. I'm an odd fellow.

You are not alone brother. 

I recall it was very common with teenagers in the early 80s.  Along with air shocks that jacked up the rear enough that you could easily see the diff cover.  I kind of want to do up a car like that, guess it was imprinted on me when I was a just a little kid.

Tyler H said:

AWSX1686 said:

I've noticed that in both my NA miatas, and also the stock diff on my ZHP all the diffs have quite a bit of surface rust. I guess they were never painted from the factory. Is there any reasoning to that besides saving a little bit of costs? Are there any downsides to cleaning the rust and painting it with POR15 or something?

ZHP diff was originally painted black and had some factory paint marks.  Mine was getting a little scaly.  I painted it with POR15 on a foam brush and it came out looking great with minimal effort or prep.  But I'm weird -- I'd rather have my suspension, brakes, and engine bay clean and detailed than the top of the car.

Yeah, thinking back the ZHP diff wasn't too bad. My miata diffs though....

wearymicrobe said:

Race car paint it white so you can see leaks when they are small and hard to find. Same with the transmission and oil pan. At least that is what I do. 

I do like that tip!

In reply to AWSX1686 :

that's why a lot of larger boat engines are white. easy to see leaks in tiny, cramped engine compartments