Do you paint your radiator? With what?

[DK540]

Twin radiators, old school stuff, not all aluminum. Fairly heavy so maybe brass and other metals. Just had 'em boiled in and out and they hold pressure. I asked the radiator shop not to paint them because I wanted to visually inspect them.

I have read that it actually assists cooling to paint them black, with a high temp paint - radiates as a black body, I assume.

What do you guys do? What paint? Is Rustoleum outdoor grill paint okay? High temp engine paint? Would you spray right through the fins or just at an angle to only coat the outside and not risk paint thickness issues?

DK

[provamo]

i have never seen an aluminum radiator painted black to improve its heat transfer performance (just saying)

[Pop Chevy]

No more than a LIGHT coat of flat black. Krylon at WalMart. Don't put anything heavy on em. Pops

[problemchild]

I would only paint them if they were hideous and visible when all the bodywork is on, if for no other reason than I want to be able to epoxy them in an emergency. You certainly do not want that goey rad shop paint.

[BeerBudgetRacing]

And definitely none of the heat resistant paints....

[DK540]

Googled the topic for a good while this afternoon:
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My Bro in law works for an outfit that fabricates radiators for NASCAR, IRL, etc. He told me they have run tests before and found that radiators painted black are slightly more efficient than if left in bare aluminum.
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http://www.molalla.net/members/leeper/coatbar.htm
From this experiment, the significant finding is:
So, in short, using a coating with a higher emissivity has a pretty drastic effect, reducing temperatures by 65 degrees Fahrenheit in the example shown.

This reduction of temperature would cause the die to run cooler, making it more efficient, and producing more light out of the flashlight.

If I had fully polished the bare Aluminum to a mirror finish, the bare one would have performed even worse in comparision.
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http://www.furukawa.co.jp/review/fr026/fr26_13.pdf
From this one, here's the scoop: From Figure 3 it can be seen that compared
with unpainted materials with an emissivity of 0.1,
FUSCOAT HS with an emissivity of 0.92 can reduce
the material temperature by about 10°C.
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And this one is just pretty darn clear about it:
http://www.engineeringtoolbox.com/em...nts-d_447.html
A bare aluminum radiator will not radiate heat as well as a painted one. A polished aluminum radiator will radiate even less heat. Ever wonder why radiators are painted black? That's because the emissivity coefficient of aluminum varies from about 0.04 for polished aluminum to 0.31 for heavily oxidized aluminum compared to 0.89 for black epoxy paint. (!!!!)
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So, as crazy as it may sound, instead of black paint being some kind of insulator and preventing the radiator from, well, radiating, it actually helps it!

But, nothing beats practical experience and I wondered if anyone had experimented, found other results, or found good paint, bad paint, or better or worse painting techniques.

It does seem, as noted earlier, that a very light coat might be best, achieving the blackness without insulating properties.

DK

[Hawke]

A million years ago when I designed radiator systems for industrial applications, we found that any paint on the core decreased its performance. There were some improvements in corrosion control if a light paint was done.

My advice - don't paint - but if you are compelled to do so, don't allow the radiator dude do it. Radiator fitters are not known for their finesse in the fine arts. Must be something to do with inhaling all those lead fumes when soldering.

Edit.
I've just seen the post above. Just remember that radiators don't really radiate, the heat is conducted by the fins and to a lesser extent the tubes, to the air flowing across the fins and tubes.

[Jeremy Soule]

I took an engineering course in Heat Transfer many years ago and also was involved at the time in open wheel dirt track racing , my car overheating during the hot summer night events... One of my sponcers was a local radiator shop!
It's well known that if you paint it black and leave it out in the sun it will absorb more raidiant heat from the sunlight, but I couldn't find any information to say that a black metal object will radiate more heat away from itself if it's in a dark place, like under the cowling of a racecar.
I too asked the repairman NOT to paint the my radiator but when I went to pick it up it was fresly painted, black of course, when confronted the repairman he said that " We have to do it to prevent corrosion" but being as I was getting the repair for free I thanked the man went back to the shop polished the top tank with steel wool till the brass glowed and ran it that way for at least 3 more seasons till I lost track of the car
Oh I see Hawke just cleared it up
So that is as far as I got with that experiment . Maybe someone with more of a science background than I can explain....

[924RACR]

Makes sense to me, leaving 'em unpainted.
Reaching back a couple of decades to my own Intro to Heat Transfer courses... in this case there should be two heat transfer methods active: conduction and radiation.

Conduction is the transfer of energy via contact, pulling it from the coolant into the metal of the rad, then from the rad into the airstream.

Radiation is the emission of heat energy via thermal radiation - electromagnetic radiation. Think red hot, white hot, etc. Happens even in a vacuum (aka black body radiation etc):
https://en.wikipedia.org/wiki/Thermal_radiation

The latter case is where the color of the object matters... but the rad is not set up to work this way; rather, it's primarily a conduction device...
https://en.wikipedia.org/wiki/Thermal_conduction

So, in this case, the paint will work similarly to a ceramic coating; it's an insulator, and slows heat transfer from the metal of the rad to the ambient air. The expectation (assumption) being that the paint has a lower thermal conductivity (essentially the thermal analog of electrical resistance) than any metal.

Recall, if you like to play in the kitchen as I do, that various pots can have inserts of aluminum or copper to ensure better, more even heating and less hot spots across the base of the pan... whereas a cast iron skillet will take a while to heat up and give even heat... but I digress...

[DaveW]

In addition to what 924RACR said, paint, even if a very thin coating, blocks some of the airflow area, so, IMO, do not paint our "convectors".

True radiators, like the old steam heat ones, can be painted, since most of their heat transfer is by radiation.

[DK540]

Thanks for the wise input from research and experience. "Convectors" will go in unpainted. I will likely ask a few teams in the pits at an IMSA race this Fall what they do, just to add to the database, but even if bare is the "err", it's certainly easier to go back and add something (paint or other coating) than to try to get it off.

DK

[provamo]

duh...................look at F1 lol

[dsmithwc04]

duh...................look at F1 lol

I just pulled up a ton of images of current and past pictures of F1 cars showing the radiators. Not a single image popped up a painted radiator of ANY color. This tells me all I need to know about painting radiators on race cars.

[R. Pare]

There are two things controlling heat transfer out of a radiator ( all other factors being equal)::

1 - The emissivity rate - which controls ONLY the infared emissions.

2 - The conductive thermal transfer rate of the surface material.


Any increase in emissivity rate will help, but only VERY slightly - single-digit percentages.

The overriding factor is the thermal transfer rate of the surface material.

The basic metal that the radiator is made from will always have a higher heat transfer rate internal to the metal that at the surface to the surrounding air, so the transfer rate of the surface material, whether it be the bare metal or some sort of coating, is all-important.

Most, if not all, paints will act as an insulator - eg - they have a lower thermal transfer rate at their surface than the metal it covers, so for us, painting the radiators is 99.99% going to be a no-no.

However, anodizing an aluminium radiator will usually increase the overall efficiency by 4-8% because of the doubling of the emissivity transfer in a forced air situation like ours radiators work in.

There may be a coating that the F1 guys use to help transfer efficiency, but I've never read anything about it. However, if they are using alu radiators, I would not be surprised if they were anodized, but would definitely be curious to know how they are doing it on already-assembled cores.

[Frank C]

As noted by Dave W above, the effects of paint on the fins and tubes are not positive in two ways. The paints adds what is an insulating effect and also can block the air flow. Neglecting the radiation, the heat transfer from the coolant to the air is controlled by a total thermal resistance which is comprised of the convection resistance from the coolant to the tube wall, the conduction resistance through the tube wall and out through the fins, and the convection resistance from the outside of the fins/tubes to the air. Paint just adds an extra conduction resistance to the sum and can degrade the convection (increase the convection resistance) by blocking air flow. I think that if you want it pretty, paint the headers (tanks) but keep the paint off the fins and tubes except for very little. I don't think the radiation or convection from the headers is very important so it doesn't matter much what you do to them.
Frank C