Sandblast radiators and...

eappleton

Hi all,

I’m planning to have the radiators in our apartment sandblasted. I’m wondering if it’s okay to leave them unfinished or clear coated. I’m guessing they would rust if I don’t have them painted, but I would like a minimal treatment. This article recommends lacquer or a clear coat that needs to cure at 350 degrees (https://www.oldhouseweb.com/how-to-advice/applying-a-clear-coat-to-a-refurbished-cast-iron-radiator.shtml). I guess I should just find a sandblasting company that can also powder coat, but ask them to clear coat and bake? But not over 451...

Any advice on clear coating radiators?

I hope this isn’t a question that has been answered elsewhere. I read these posts first:

https://heatinghelp.com/blog/how-to-paint-an-old-radiator/
https://forum.heatinghelp.com/discussion/181395/radiator-refinishing

Thanks!

TomTA

From what I've heard powdercoating radiators can cause problems. The high heat used to cure the powdercoating can burn up the paper/fiber gaskets between radiator sections for some types of radiators. The local used radiator emporium has big signs everywhere saying DON'T POWDERCOAT YOUR RADIATOR - IT VOIDS THE WARRANTY.

We had a few done recently, and just sprayed with silver metallic or bronze metallic Rustoleum after sandblasting. The silver metallic is almost indistinguishable from the raw sandblasted cast iron. Not sure if that would survive steam radiator temperatures, but so far so good for hot water system.

Be careful with the choice of sandblaster, too. If you've got the old Victorian decorative ones a too-aggressive abrasive or a heavy-handed worker can really tear up the decorative details.

eappleton

Thank you. That worries me as well. I've read that the powder coating is done at 400 degrees and since paper burns at around 450, then maybe it's okay. But that seems too close for comfort to me. 

I'm trying to avoid metallic finishes since that purportedly reduces heat output. Though I like the look. 

Thanks for the warning in sandblasting. The radiators are from 1928 or so, but they're utilitarian. Nothing decorative. 

Maybe I'll just get them sandblasted and spray clear coat with a high heat rattle can. This for example? https://www.homedepot.com/p/Rust-Oleum-Automotive-12-oz-High-Heat-Matte-Clear-Protective-Enamel-Spray-Paint-260771/205751531

ChrisJ

Why does everyone claim heating in an oven will damage them but no one is concerned about heat from sand blasting?

What's with the paper gaskets?  Where were these?

acwagner

If they're from the '20's, they're probably threaded nipple connections and don't have gaskets. Do you know the manufacturer and model of your radiators?

Ecorad refurbishes radiators and one of the finish options is a clear coat, so some companies do it.

AdmiralYoda

I really wouldn't worry too much about metallic finishes and the potential for reduced heat output.

Sure, it's a thing but really not that much of a concern.  Most of my radiators have 20 layers of oil and latex housepaint and they get screaming hit.

In the summer I'm going to bring one out to the driveway and hit it with some aircraft remover and a pressure washer.  Then spray it with some high temp enamel.

ChrisJ

AdmiralYoda said:

I really wouldn't worry too much about metallic finishes and the potential for reduced heat output.

Sure, it's a thing but really not that much of a concern.  Most of my radiators have 20 layers of oil and latex housepaint and they get screaming hit.

In the summer I'm going to bring one out to the driveway and hit it with some aircraft remover and a pressure washer.  Then spray it with some high temp enamel.

20 layers is an issue but it's not the same as the actual color of the radiator.

The actual color makes a significant different in Infrared output.

eappleton

@acwagner I'm not sure about the make and model of the radiators. The logo is SR? SN? 

KC_Jones

You shouldn’t need high heat paint, high heat is a grill or exhaust manifold on a car.  Regular paints will easily hold up to radiator heat.  The surface of a car on a hot sunny day will easily meet or exceed the temp of a rad, especially a black car.  Generally anything beyond about 300 starts getting risky for regular paints.

I’ve read about the elusive paper gaskets on here before.  Even if they are real, no way are they still surviving after 100 years.  Also what radiator construction were they used on?  Push nipples shouldn’t need them, right/left threaded nipples shouldn’t need them, so what did use them?

I’m genuinely curious about this, hoping for an education (not speculation) on this.

I wouldn’t hesitate to powder coat knowing what I do about radiators and cast iron.

AdmiralYoda

ChrisJ said:

AdmiralYoda said:

I really wouldn't worry too much about metallic finishes and the potential for reduced heat output.

Sure, it's a thing but really not that much of a concern.  Most of my radiators have 20 layers of oil and latex housepaint and they get screaming hit.

In the summer I'm going to bring one out to the driveway and hit it with some aircraft remover and a pressure washer.  Then spray it with some high temp enamel.

20 layers is an issue but it's not the same as the actual color of the radiator.

The actual color makes a significant different in Infrared output.

I'm always up for learning something new.  Let's say I was to start from scratch and plan to repaint...what's the optimal color for the highest heat transfer and what's to worst?

My wife wants white, I want silver.  And btw I was exaggerating a wee bit on the 20 coats, but you get the idea.  👍

KC_Jones

Flat black offers the highest radiant transfer.  I would never do a flat color though as it would make them even harder to clean.  Gloss black would look awesome with the right decor.

ChrisJ

KC_Jones said:

Flat black offers the highest radiant transfer.  I would never do a flat color though as it would make them even harder to clean.  Gloss black would look awesome with the right decor.

If I ever get around to it I plan on flat black.

PC7060

KC_Jones said:

I’ve read about the elusive paper gaskets on here before.  Even if they are real, no way are they still surviving after 100 years.  Also what radiator construction were they used on?  Push nipples shouldn’t need them, right/left threaded nipples shouldn’t need them, so what did use them?

What KC said, I’ve got four bolt type radiators that I will be having sandblasted and painted. The radiators are 1930-40s era, is this when paper gaskets were prevalent?

deyrup

You are looking for paints with a high emissivity rating. https://www.thermoworks.com/emissivity-table most paints have high emissivity, although a lot of radiators were intentionally painted with paints that have low emissivity to lower heat radiation.

Canucker

KC_Jones said:

Flat black offers the highest radiant transfer.</blockquote


Close but not quite.

https://heatinghelp.com/heating-museum/does-the-color-of-a-radiator-matter/

acwagner

@eappleton the maker's mark is usually on the main casting. Check the around the bottom taps where the pipe comes in.

Your radiators look like the push nipple type, which have threaded rods that go the whole length.

I did a quick internet search and it appears that gasket style radiators are more common in Europe.

https://www.castironradiators.ltd.uk/gasket-research-165-w.asp

TomTA

AdmiralYoda said:

In the summer I'm going to bring one out to the driveway and hit it with some aircraft remover and a pressure washer.  Then spray it with some high temp enamel.

I started out with aircraft stripper and a wire brush, but quickly decided that paying $40 for sandblasting was a heck of a lot more sensible than spending several hours doing a miserable job, with a poorer result, with the stripper probably costing at least half that amount. It's rare that I make a really good decision, so I'm relishing that one.

AdmiralYoda

@TomTA wow, I didn't know it was that inexpensive.  Just have to find a sandblasting place near me.

ChrisJ

Seems like the media alone would cost at least $40 alone for sand blasting.

acwagner

Plus transporting to the workspace and back.

TomTA

AdmiralYoda said:

Just have to find a sandblasting place near me.

I live in a rural area, so prices may be less than what they'd be in a city. Powdercoating places do sandblasting prior to powdercoating; find one and tell them you want "blast only". If that doesn't pan out, an auto or truck body place can probably point you to someone. Be sure to put plugs in the water inlet/outlet so you don't get blasting media inside.

Dave in QCA

The LOGO on your radiator is U S. That is the U. S. Radiator Co. They always used push nipples and rods. For paint, I have become fond of Rustoleum spray paint in a color called Oiled Bronze. It is very dark, but not black. The sheen is not flat, but certainly not gloss either. Probably best described as satin, or semigloss. It performs perfectly on a steam radiator that has been sandblasted. I've had good luck over painting without sandblasting too.

AdmiralYoda

Any benefits from using high temp enamel such as the products for coating engine blocks and such?  

The price is about the same for me, wondering if the high temp enamel might be a bit better.

KC_Jones

AdmiralYoda said:

Any benefits from using high temp enamel such as the products for coating engine blocks and such?  

The price is about the same for me, wondering if the high temp enamel might be a bit better.

Not in my opinion, and for reference in the circles I travel, we paint engine blocks with regular automotive paint. I have a close friend who is a professional painter and he uses basecoat/clearcoat on them. I've seen complete engine blocks ground smooth to remove all casting marks from the sand mold and then painted, again with regular automotive paint.

212° on a steam radiator isn't that hot, it only seems hot because you interact with it so often and it's above a typical comfort level for human skin. In the grand scheme it isn't high enough to warrant any special treatment, many on here use regular (good quality) house paint with good results.

PC7060

@Dave in QCA  - "The LOGO on your radiator is U S. That is the U. S. Radiator Co. They always used push nipples and rods." 

The four 3-1/2” x 28 x 44 radiators I bought from a local salvage place are Weil McClain with four long bolts. I had no idea WM made the big radiators. I’ll be having these blasted and painted in spring. Not sure if I’ll go with powder coat or spray coat.

eappleton

Thanks, @acwagner and @Dave in QCA. Funny that I was reading the logo upside down. So powdercoating would be fine on my radiators. Or a spray can of Rustoleum. I’ll decide in the spring when I can pull them out. @TomTA, thanks for the tip to plug the water intake. I never would have thought of that.

Thanks for all the advice, everyone.

SlowYourRoll

KC_Jones said:

Flat black offers the highest radiant transfer.  I would never do a flat color though as it would make them even harder to clean.  Gloss black would look awesome with the right decor.

I was thinking about trying to maximize radiator efficiency by going with a higher emissivity color, until I read about why all the old radiators are painted in metallic silver (which has low emissivity). i don't need to tell the whole Spanish Flu story cause other people on here and in the books tell it better, but the short version is that the radiators from the 20s were originally sized to heat the house with the windows open all winter, so once that fell out of vogue everyone had oversized radiators in their houses, and they started painting them with lower emissivity paint on purpose in order to be able to control the heating of the house much better.

so if your radiators are oversized to begin with, and then you paint them with the highest emissivity paint you can, yes they will heat up a room more quickly, but they might work so fast you end up short-cycling the boiler. and with modern boilers working most efficiently when they aren't short-cycled, you might end up with a system that heats quick but is less efficient overall.

the speed that the radiators heat up could cause other problems in terms of overshooting the target temp in the control system. the goal is steady, even heat. this gets into control theory a bit, but the real-world analogy they always use is elevators. you can speed up your elevator, but it's going to overshoot the floor and have to sink back down a couple centimeters once you get to your floor. in terms of thermostat feedback loops it means instead of staying in a nice window of, say, 70-72, you'll be shooting up to 74 and then cutting off and short-cycling.

kinda counterintuitive at first, but it's all there, and there's a very good reason all the radiators in my house are painted that silver metallic color

hot_rod

Rustoleum does have an ultra hard finish radiator paint product, not sure about color choices 

SlowYourRoll

just a quick follow-up to my last post, this is the graph from control theory that really hammers home the importance of getting everything working together, and how optimizing the efficiency of one part of a system might not optimize everything else. so imagine the graph is plotting temperature over time. seems like the top curve would be the most comfortable, since it overshoots your set temperature fast, then undershoots it a little less, but overall the room temp would get comfy faster than the other two. that curve represents oversized and high emissivity (i.e. matte black) radiators. the problem is, the only way for the temp to start coming back down after it overshoots is to shut off the boiler. and then once the temp drops to that next valley, only way to raise the temp again is to fire up the boiler again. that's short-cycling. so as counter-intuitive as it may seem, the ideal isn't maximum efficiency radiators, but rather radiators that emit heat in balance with the rest of the system. and since everyone oversized their radiators prior to the 30s to heat a house with open windows all winter, the way to get to this optimal system is to paint them a lower emissivity color to offset that.




(also, this only applies to radiators that were oversized. if you were to install a completely new steam system today you could definitely use the highest emissivity paint you could find, as long as you sized your radiators accordingly)

eappleton

@SlowYourRoll Really interesting explanation. I definitely see how we should be careful about using high emissivity without being strategic. It seems that there could be situations where it is appropriate though. For example, I'm planning to remove a 6-column radiator (photo above) and replace it with a 5-column radiator we have in storage. I could calculate the difference in EDR between the radiators and then estimate what effect high emissivity paint would have. Maybe the two radiators would have a comparable EDR? If they did, then it wouldn't affect the whole system negatively, right?

It seems that the takeaway is that we should be strategic when choosing paint. We don't want to increase the EDR of a radiator unless that is what is needed by the room. 

I should mention that I'm thinking about repainting radiators in one apartment out of 25 total, so it seems like it would have minimal effect on the whole system unless it becomes a trend all my neighbors pick up. 

PC7060

Or you could put TRV on the units and tune each room to taste.



Works very well for Hot water hydronics although I cant speak for Steam Systems

SlowYourRoll

eappleton said:

@SlowYourRoll Really interesting explanation. I definitely see how we should be careful about using high emissivity without being strategic. It seems that there could be situations where it is appropriate though. For example, I'm planning to remove a 6-column radiator (photo above) and replace it with a 5-column radiator we have in storage. I could calculate the difference in EDR between the radiators and then estimate what effect high emissivity paint would have. Maybe the two radiators would have a comparable EDR? If they did, then it wouldn't affect the whole system negatively, right?

It seems that the takeaway is that we should be strategic when choosing paint. We don't want to increase the EDR of a radiator unless that is what is needed by the room. 

I should mention that I'm thinking about repainting radiators in one apartment out of 25 total, so it seems like it would have minimal effect on the whole system unless it becomes a trend all my neighbors pick up. 

yes, that is exactly what i was getting at, and yes there would be situations where it could be appropriate. seems like you have a good handle on how it all fits together. in fact, assuming the system worked the way you wanted beforehand, if all you did was swap in a smaller radiator, painting/finishing the smaller radiator to match the EDR of the old one would be a great way to ensure everything continued as it did before.

in fact, the heat from a radiator can only be in the form of (1) conduction/convection (where conduction is convection without air movement), or (2) radiation. conduction/convection is largely governed by surface area. surface area matters for radiation, but the emissivity of that surface comes into play. so downsizing but increasing the emissivity, you're basically replacing conduction/convection with radiation. and in terms of how the two feel, radiation is that warm glow you get lying on the beach in the sun. the system would work roughly the same between the two, but you'll be getting more of that warm glowing feel you get from steam radiators that you just can't get from a forced-air system. so overall what you describe sounds like a perfect example of when to go higher emissivity.

Dave in QCA

Here is a little radiator I had sandblasted and painted with rustoleum oiled bronze. The radiator is a 1910 US Radiator,  Grecian model. 

PC7060

Nice work Dave, radiator looks great! Now I’ve got to ask what’s behind the access panel!  I’m nosy that way  

Grallert

KC_Jones said:

You shouldn’t need high heat paint, high heat is a grill or exhaust manifold on a car.  Regular paints will easily hold up to radiator heat.  The surface of a car on a hot sunny day will easily meet or exceed the temp of a rad, especially a black car.  Generally anything beyond about 300 starts getting risky for regular paints.

I’ve read about the elusive paper gaskets on here before.  Even if they are real, no way are they still surviving after 100 years.  Also what radiator construction were they used on?  Push nipples shouldn’t need them, right/left threaded nipples shouldn’t need them, so what did use them?

I’m genuinely curious about this, hoping for an education (not speculation) on this.

I wouldn’t hesitate to powder coat knowing what I do about radiators and cast iron.

The only radiators I've found them had threaded nipples rh/lh threaded. They do seem to hold up well over the decades.

Dave in QCA

PC7060 said:

Nice work Dave, radiator looks great! Now I’ve got to ask what’s behind the access panel!  I’m nosy that way  

Ha Ha! Me too! The panel provides access to the plumbing of a bathtub. This became necessary when the mansion was converted to apartments in 1928. In its original 1909 configuration, this space was the butler's pantry and this little baby radiator was under the dish washing sink.

PC7060

@Dave in QCA -  it’s funny the Way foot prints get left behind by the previous owners and tenants. My old house was a boarding house at some point in the 40s and 50s and someone thought it would be a good idea to punch holes through into the chimney to add two potbelly stoves. I ran across the patches doing renovation on the property so called out a chimney inspector.   

The chimney sweep warned me that punch through ruin inner flue liner, but after he looks at chimney, he laughed and said no harm, your chimney never had a liner installed! He said it was not common but some of the old builders were know to cut the occasional corner.  

Steam_Gus

25 years ago without the benefit and insights of DH, I had our 11 radiators sandblasted (primarily as lead paint abatement for our young children) and spray painted with a bronze low luster BenMor metallic enamel paint that has held up remarkably well over the years. Tuned up the system with Maid o Mist vents and sealed all the valve stems and couplings to reduce water loss to acceptable levels. Been very comfortable and happy all these years. 

Can't say I've noticed short cycling.  But then again what would constitute short cycling by a system spec, how many firings per hour on how cold a day? Can someone please elaborate? 

1923 colonial (~1400 sqft) with uninsulated exterior walls. Was oil fired Weil McClain, now gas fired Utica.

gunn308

Or you could put TRV on the units and tune each room to taste.



Works very well for Hot water hydronics although I cant speak for Steam Systems

I have 40 zone valves like that on a 2 pipe steam system in our church they work fine.

wam525

TomTA said:

From what I've heard

We had a few done recently, and just sprayed with silver metallic or bronze metallic Rustoleum after sandblasting. The silver metallic is almost indistinguishable from the raw sandblasted cast iron. Not sure if that would survive steam radiator temperatures, but so far so good for hot water system.

I agree. We found a guy who had a portable sandblaster and lugged several of our radiators outside for blasting. Then spray painted them with metallic Rustoleum. They still look great after 30 years.

JoeHammes_2

My crew and I have repainted hundreds of radiators during the course of old-house projects and almost always use Rustoleum oil paint with very good results. The important thing is of course prep - if they have multiple coats of thick paint, then we take them out for sandblasting. If they are fairly clean, we power-wash and wire-brush until clean and smooth. Then plug and pressure-test, then paint with our HVLP sprayer. The trick is to thin the paint down to an almost watery viscosity - this allows me to shoot continuously at every angle to get everything coated without too much buildup. When the viscosity is right, excess paint will drip off the bottom but won't show visible runs or sags. Then we either restore or replace the valves and fittings, add new bleeders, and install. I know of 40-plus that are still clean and in great shape after over 20 years - including 8 in my own house. Mostly hot water, but several are in steam systems with no issues.