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Old 05-20-2007, 08:40 PM
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Painting over powdercoat??

Guys,

I know that this is going to sound like heresy, but I need to paint over some powdercoating!!

I am looking at Suspension Techniques Front and Rear Sway Bar kit (#52245) for my G body donor chassis. Only problem is that the darn bars are GREEN (yes, I said GREEN) powdercoated, and I don't want GREEN sway bars!!

Any advice re can I paint over the powdercoat and how to do it (prep-wise) will be greatly appreciated.

texastomeh

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Old 05-20-2007, 08:58 PM
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Powder coating is paint powder that has been melted onto the surface it is coating. Rough up the powder coat with a Scotch Brite red pad and paint it, it will be fine.

Vince
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Old 05-20-2007, 09:33 PM
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Thanx for the reply Vince!!

That was what I was thinking, but wanted to run it by the experts just to make sure!

texastomeh
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Old 05-20-2007, 10:13 PM
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I scuffed some coil springs with 400 grit that were powder coated and painted them with a rattle can graphite colored wheel paint about 4 years ago and they still look good.
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Old 05-21-2007, 06:58 AM
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I've painted over powdercoat many times and never any failures, usually scuff with a red/maroon scotchbrite and shoot one sealer coat of epoxy for adhesion then my paint.
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Old 05-21-2007, 08:52 AM
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Bob, That brings up something I have been meaning to ask-how would epoxy work in a situation like on those springs where there is a lot of flex involved?
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Old 05-21-2007, 09:09 AM
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Powdercoat as a primer seems to be the new thing. Hot Rod Magazine had the entire body of their F-Bomb Camaro powdercoated prior to any bodywork or paint.
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Old 05-21-2007, 09:57 AM
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Originally Posted by joe_padavano
Powdercoat as a primer seems to be the new thing. Hot Rod Magazine had the entire body of their F-Bomb Camaro powdercoated prior to any bodywork or paint.
Now, that scares me - you have to bake powder coat for up to an hour at 375 - 400F to flow it out. The body metal will start to move around at that type of temp and make all kinds of permanent waves in panels. While I have forgotten my physics, steel moves - a lot.
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Old 05-21-2007, 01:27 PM
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Originally Posted by Irelands child
Now, that scares me - you have to bake powder coat for up to an hour at 375 - 400F to flow it out. The body metal will start to move around at that type of temp and make all kinds of permanent waves in panels. While I have forgotten my physics, steel moves - a lot.
Well, the expansion rate of mild steel is about 0.000008 inches/inch/degree F. Assuming a 400 deg F cure on a 15 ft long body, that's a total expansion of 0.47" if you started at 70 deg F.

Of course, that's really meaningless, since if you heat the body uniformly in an oven, and the part is entirely made of the same material (which it is), then the whole body expands uniformly and there is no warping. The problem is when you're welding a patch panel and locally heating the weld to well in excess of 1000 deg F. THAT'S when you get warpage.
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Old 05-21-2007, 02:14 PM
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Sorry but your analysis doesn't work for me. The car is not a single sheet or bar of steel but made up of many components that will expand at different rates.

I could come closer to agreeing with you IF all of the steel were of the same thickness that it could return to the as original shape. Unfortunately, brackets and bracing are also of varying thickness, are welded, rivited or bolted to an underlying framework/floorpan/framework, resisting panel movement at different rates. This is where you will get deformation or warping. Additionally, as the steel is die formed formed, it is thinner/thicker in various areas. While I am not familiar with Hot Rod's Camaro, if it is after an market component car, body panels are most likely made up of an extremely soft draw quality steel and will probably move even more. If Hot Rod made it work, my guess is that they used a special, very low temp powder coating material that will not withstand the rigors of the environment without a topcoat of paint of some sort. Then if this is the case, it brings up an additional area of concern - will it stay "stuck" to the panels in occasional 110F CA weather or fail, along with the top coats.

Dave
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Old 05-21-2007, 03:16 PM
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Quote:
Originally Posted by Irelands child
Sorry but your analysis doesn't work for me. The car is not a single sheet or bar of steel but made up of many components that will expand at different rates.
First, the car body shell IS made up of one material (mild steel) that all expands at the same rate. It doesn't matter how many pieces you have or how they're welded together. If you heat them in an oven with circulating air such that the temperature inside the oven is consistent, then the all expand at exactly the same rate. That's the definition of coefficient of thermal expansion. If you have different materials (some aluminum, some steel) then you are correct, you will get a differential thermal expansion. That's what goes on inside an engine where you have aluminum pistons and an iron block.

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I could come closer to agreeing with you IF all of the steel were of the same thickness that it could return to the as original shape.
Doesn't matter how thick the steel is. The length will grow by the same amount if they are heated evenly to the same temperature. Again, that's how the coefficient of thermal expansion works. Yes, if you have a very thick part and a thin piece, the thick part won't heat as fast and thus won't expand as quickly, but you'd need a significant difference in thickness, like a 1/16" piece of sheet and a 4" thick bar. Even then, if you leave them in the oven long enough so the temperatures stabilize, the expansion will be the same. Within the variation of sheet metal thicknesses in a car body shell, the differences won't matter.

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Unfortunately, brackets and bracing are also of varying thickness, are welded, rivited or bolted to an underlying framework/floorpan/framework, resisting panel movement at different rates.
If the panels are all steel, they all expand at exactly the same rate.

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This is where you will get deformation or warping. Additionally, as the steel is die formed formed, it is thinner/thicker in various areas. While I am not familiar with Hot Rod's Camaro, if it is after an market component car, body panels are most likely made up of an extremely soft draw quality steel and will probably move even more.
Nope. An original second gen F-body.

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If Hot Rod made it work, my guess is that they used a special, very low temp powder coating material that will not withstand the rigors of the environment without a topcoat of paint of some sort.
Nope again. Normal 400 degree cure.

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Then if this is the case, it brings up an additional area of concern - will it stay "stuck" to the panels in occasional 110F CA weather or fail, along with the top coats.

Dave
I understand how you might be concerned, but you probably should read the article and look at the photos before jumping to conclusions. If you don't trust it, don't use it. I certainly have no first hand experience with the technique, but the magazine spells it out in pretty complete detail. Also, if you go to various powder supplier sites, such as DuPont, you will see that they are also selling their powdercoating materials for use on OEM undercoats.
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Old 05-21-2007, 03:24 PM
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I won't comment from an auto body repair standpoint since I am no pro but from a metal workers perspective I have to agree that IF the body is heated evenly and then cooled evenly then warpage should not be a problem. I would have to wonder however about other parts being affected such as rubber mounts etc, not saying it is a problem just thinking maybe?
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Old 05-21-2007, 03:47 PM
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Quote:
Originally Posted by joe_padavano
First, the car body shell IS made up of one material (mild steel) that all expands at the same rate.....
Joe, I wont go further except to say different thicknesses of steel plus being restrained by brackets, there will be differences in the rate of expansion - the coefficient will remain the same. The rate is what will cause you problems

Hot Rod magazine, along with many other mags has, IMHO, very little credibility left. But that's for another post
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Old 05-21-2007, 03:52 PM
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plus being restrained by brackets
Good point, that is something to consider.
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Old 05-21-2007, 06:29 PM
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Originally Posted by oldred
I won't comment from an auto body repair standpoint since I am no pro but from a metal workers perspective I have to agree that IF the body is heated evenly and then cooled evenly then warpage should not be a problem. I would have to wonder however about other parts being affected such as rubber mounts etc, not saying it is a problem just thinking maybe?
This has to be a bare steel shell. Any rubber, plastic, aluminum, etc. must be removed.
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