Will White Paint Under Black Paint Reduce Temperature From Sunlight. 1

[Teardrop]

Hope you guys can solve a problem that I have been struggling with for a while.

I have some fiber-glass parts that I made from epoxy (west system) years ago. I have spent hours post curing them in the sun but they still show some print-through when they get hot in full sun. It is not practical to put them in an oven to take the temperature higher. I spoke to technician at west system who said that it probably would not be possible to stabilize the part beyond what I've already done even with an oven.

The parts are painted gloss black and have been polished to a mirror finish so show every flaw. I recently spoke to a paint expert in the marine industry who told me that they have had success painting parts white first then painting final color over the top. He says this reduces the temperature in the sun by as much as 10 degrees F.

Obviously this would be a great deal of extra work and materials for me but I would be willing to do it as the print through seems to appear at the upper end of the temp the part gets to (around 160*F).

Can anyone confirm this? The paint does not have to be a specific type, it is the color that makes the difference.

Thanks for any replies.

 

[TugboatEng]

Print through like you see texture from the glass? Try applying a thin layer of fairing compound to the surface using West System 410.

 

[Teardrop]

Thanks for reply.
These parts already have many layers of fairing compound plus many coats of high build primer. Also they are large compound curves so it is very difficult to put compound on certain areas without affecting the shape. Plus the parts were originally made up with a screen mat on top specifically for epoxy lay-ups to prevent print-through.

 

[Compositepro]

Print through is caused by shrinkage of resin during cure and also the difference in CTE between resin and fiber. The resin in the weave openings of the fiberglass fabric expands and contracts with temperature more than the fibers and this pattern shows on the surface. Gel coats and fairing compounds contain mineral fillers which reduce the CTE of the resin. You can make the surface smooth at room temperature by sanding and painting but at elevated temperature the resin under the paint will still expand more than the fibers and you start seeing the weave pattern print through the paint.

You can probably greatly reduce the visible print through by sanding your part smooth at some intermediate intermediate temperature, say 120F, so you see half the amount of print-through that you see now. But, you will probably see some print-through at room temperature.

A surfacing film or gel coat applied to the part prior to cure will help quite a bit as well.

 

[Teardrop]

Sorry guys I think the title of my thread is a bit misleading. I do appreciate your replies but I'm not interested in refinishing the part or making new parts. I have way too much time in these at this stage. I was hoping to hear from any paint experts that could confirm the possibility of reducing final temp. caused by the sun by painting white under the finished coats.

Thanks.

 

[TugboatEng]

I would say no. White paint under black paint would perform identically to two coats of black paint.

 

[Teardrop]

Thanks again for reply TugboatEng.

I was also very skeptical and questioned him at length about this and he seemed to have experienced the same problem I had and was pretty sure it would work.

I don't think I'll be trying it though unless I can get some kind of confirmation from someone else. It's just too much work.

By the way I've just edited the title of this thread to make it clear what info I'm looking for.

 

[IRstuff]

Sounds like someone needs their critical thinking skills revisited and rebooted. Black paint is opaque for all intents and purposes, so its transmission is close to zero, so what possible purpose will the white paint have? It is possible to get materials that are highly reflective in the visible and highly emissive in the infrared, but that's not a paint that you buy at Home Depot.

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[Teardrop]

I agree. My thinking was the only way this might 'work' is if the white paint lightly altered the shade of black but I'm no paint expert so wanted confirmation elsewhere.

Thanks for replies.

 

[moltenmetal]

If you want to block radiant heat transfer, you need an air gap- or better still, a vacuum- between your hot surface and the one yu're trying to keep cool. If the two surfaces are bonded together and conduction can happen, you're done.

If you had a black surface separated from a white surface by an air gap, the fact that the 2nd surface to see the heat was white might make some difference. But we agree that a layer of white overcoated by a layer of black is the same as two coats of black.

 

[IRstuff]

A cavity will thermally equilibrate, from air conduction, if nothing else. Nevertheless, the radiative transfer in a view factor 1.0 situation will likewise equilibrate, particularly since the infrared emissivity of most paints, regardless of color, are pretty high (

https://www.thermoworks.com/emissivity-table).

This is why cavity blackbodies have emissivities that are close to 1.0. Thermos bottles have highly reflective surfaces facing each other across the vacuum gap to minimize radiative transfer.

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[hatsegal]

Teardrop,
Is your question about the temperature during the process or at the final state?
Color does have an impact on heat transfer rate by radiation via the "emissivity" factor (e) with Black being the most effective color (e =~1) and white being the least effective (e closer to 0).
So, in theory, if you consider the in-process temps, then white color (during application and curing period) will result in theoretically lower net heat transfer during that curing time than a black intermediate coat would (assuming both black and white intermediate coats have the same curing time).
As a result, and all other things being equal, your epoxy body should be "theoretically" at a lower temp at the end of curing of the white intermediate coat vs. if the intermediate coat was black (again all other things being equal). In real terms, the extent of the temp difference will depend on many factors and is the kind of thing that PhD theses are researched and written about.


If you were to apply the intermediate coat (black or white) and subsequently the top coat (black) in an infinitely small amount of time (if that were practical), then the intermediate coat being white or black would be of no consequence assuming the material and thickness of this intermediate coat (white or black) is the same. This is because the thermal resistance offered would effectively be the same as radiation (and therefore color) doesn't factor as much with respect to this layer in the heat transfer problem from "sun/environment" to epoxy base, under this condition.

Fundamental food for thought.

Cheers, hatsegal
"Think first"

 

[IRstuff]

Again, note that visible color isn't necessarily correlated with LWIR emissivity:

https://www.thermoworks.com/emissivity-table

White paint's emissivity is pretty high.

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[hatsegal]

IRstuff,
You are focused on IR and may not be giving due attention to Visible Light (from which color is derived) as another important contributor to thermal radiation and radiation heat transfer.
How an IR thermal sensor works does not correlate directly to how "radiation heat transfer" works.
The following is a useful experiment:

https://www.sciencebuddies.org/scie.../radiant-energy-absorption-colors#background.

Cheers, hatsegal
"Think first"