Powder Coat--Problems

[mdsalemi]

Hi everyone,

I got a PM from a member here, and friend, about some powder coat issues. I thought I'd share.

The member was so impressed with the appearance of powder coating the air filter canister, that he went ahead and powder coated the valve cover. The results were dreadful--bubbles, pits, rough surface, runs…and he asked me why. This is my reply:

Well, your powder coat guy is correct. However, if he is a true powder coat professional, he should know in advance precisely how to deal with the material he is coating. If you see the attached article, you'll see what I mean. Either he doesn't know about it, or failed to tell you, or both…in any case that takes him a step down on the rung of professionals…sorry to say.

In a nutshell, certain alloys and metals present interesting powder coat challenges. My powder coater, http://www.exoticcoatings.com knows all about these things (as he should!) and had he been given this project, he would have prepared it properly which would have included a long time in the pre-bake oven to do the "out-gassing".

I've had all my aluminum wheels (I have those pressed aluminum wheels) powder coated, which isn't as much of a challenge as a casting. Nonetheless if you want it done right send the valve cover to these guys here in Michigan. They'll do it right.

I didn't understand the blasting with 180 grit aluminum oxide; if this was done BEFORE the powder coating, I'm saddened to report to you that this only complicated the process. The first aluminum wheel I had done was done by a professional wheel refinishing operation in Indiana, as a favor to me through a friend at Hayes (as in Hayes-Lemmerz, the original manufacturer of those wheels). They blasted the wheel, and in the process added a rough surface to the wheel when it was smooth to begin with. They also, in the process, blasted out the wheel size, part number, and company logos which had been stamped into the wheel! I eventually sold those 2 wheels to another SL113 member with full disclosure. When I did subsequent wheels at Exotic here in Michigan, I asked them about that and they said they would chemically treat the wheels, not blast them thus retaining the surface integrity. That they did and they did a flawless job. So, if your powder coater did a media blast on your valve cover before the coating, he compromised the integrity of the smooth surface. If he is telling you he needs to blast it after the fact he's clueless, I'm afraid.

The article I referred to in my reply is too big to attach here, but here's a link: http://alloynet.com/uploads/Powder_Coating.pdf

[jameshoward]

Michael,

Interesting. There is quite a body of 'evidence' to suggest that high temp powder coating and off gassing of alloys can be quite dangerous. Most 113s will run on steel, so not a problem. But modern alloys heated to off gas and coat can fail critically. I think anyone considering coating alloys must do their homework.

[mdsalemi]

James, as far as this one item--valve cover--goes, I don't think one needs any "high temp" coating. There are high temp [powder] coatings that are used on items such as manifolds and generally use a ceramic material, not a plastic resin. The issue in this case was the powder coater simply not having enough experience with alloys, and thus not preparing the item properly. I would suggest that our valve covers can surely stand a de-gas operation since it isn't much different than the final oven bake of powder, it just happens before and is part of the preparation process. Further, using mechanical media for blasting on aluminum is just wrong, unless you specifically want a change to the surface.

Anyone contemplating such powder coating--on aluminum--should familiarize themselves with some of these issues (that was just one article available online) and choose a powder coater who demonstrates knowledge and experience in the material you want to coat.

[jameshoward]

Michael,

I disagree, and a look through the net will provide examples. I'm not talking about high temp coatings for manifolds. The actor heating alloys for normal not high temp powder coating is seen as a risk.

Just to be clear.

I will think and study carefully before coating more wheels.

JH

[mdsalemi]

James,

There's a huge difference between a structural part (such as a wheel, or a suspension part) made of a lightweight alloy, versus a part such as a differential cover, a valve cover, or sump/sump cover.
Most parts, particularly older auto parts, the owner or powder coater has no idea of the precise alloy, nor of whether or not that alloy can withstand the slight increase in temperature for a de-gas process. If the de-gas temp would cause some structural change to the casting that might compromise the strength of the part, this would be a problem on a structural part, and less of a concern on a non-structural part. While there are all kinds of issues with cast-alloy powder coating, there are surely more successes than failures. The successes I suggest, are in the hands of people who know what they are doing and have the experience to back up their decisions.
I trust my powder coater, and his showroom has a larger variety of powder coated parts of all kinds, of all metals, than most of us would see in a lifetime. Yes there are plenty of cast alloy parts on display, with a finish that is flawless.
There's also a hugh difference between cast-alloy wheels and pressed wheels. The metal is totally different; castings are porous and pressings not; it's the porosity that is the cause of the powder coat failures.

When I wanted to have my coolant overflow tank (brass) powder coated, they were immediately suspect--even though several of our members seemingly had theirs done without incident. One of the things they did first was remove the paint chemically, then a cleaning and pre-heat process; the preheat is similar to the de-gas but done at curing temp, not elevated like a de-gas. During this preheat, the solder started to flow out of the seams, and more importantly, perforations appeared in the brass tank; the result of perhaps, acidic coolant over the years that had weakened the structure. The powder coater didn't charge me for the stripping and cleaning but said he did not recommend powder coating this tank.
I found an old-fashioned radiator repair shop, and they re-soldered the tank seams, re-soldered the neck fitting, and repaired the perforations, all using solder. Some people suggested using hi-temp silver solder or brazing but the guy who did the repair suggested this was a bad idea; the high temperature required for either would warp and or deform the thin brass that the tank is made of.

I took the advice of experts. When the tank was all repaired (and pressure tested) I brought it to my body guy and they put a beautiful paint job on it. The paint remains beautiful, and was better than the first paint job. It matches the powder coated parts in my engine bay, such as the air cleaner canister and fan shroud. 

[glcg123]

I question why anyone would want to powder coat any part when heat has to be used to cure. For wheels a good walnut shell blast followed up with a good primer and paint will duplicate (and may improve upon) what came from the the factory. Ten years ago I had a choice for my wheels and stayed with the aforementioned process. No problems whatsoever. They still look great.

The new catalytic paints cure extremely well and are very durable.

Now if I just keep the brake dust off the front wheels...

George
1965 230SL

[mdsalemi]

I question why anyone would want to powder coat any part when heat has to be used to cure.

George, it's quite simple, really. Powder coating produces a harder, more durable surface than paint. That's why it is used. It is also more expensive to do, which is why it often isn't used in certain places on a car.

Many of the formerly painted surfaces of my car have been powder coated with the same kind of color and surface sheen as the original paint. I do this when I have the opportunity; such as when I removed the radiator, and had to remove the shroud and air cleaner canister. Those were powder coated while I waited for the radiator to be returned from SL-Tech.

Having small parts powder coated is what the traditional powder-coat houses around here do; they are used to someone coming in with one part to be coated. To get a good paint job (if you are not doing it yourself) requires going to a body shop. While they certainly will do it for you, they don't traditionally specialize in one-off, small parts.

I paid precisely USD $70 each for my aluminum wheels to be chemically stripped and cleaned and then powder coated in black. I suspect that taking the wheels to a body shop would not have been any less. You probably don't want to blast, even with walnut shells, an aluminum wheel. Chemical stripping and prep is the way to go. Every reputable powder coat shop has the means and equipment to do this, but not all body shops do.

As for heat, well, factory applied paint is typically cured in an oven, often at temps equivalent to powder coat...however you won't find these kinds of temps in a body shop as they are way too high!

[glcg123]

My wheels were done by a shop specializing in wheel repair and refinishing. They were steel wheels so no issues with the walnut shell blasting.

George

[Raymond]

Michael, we have a comparison test going on.  Several years ago I purchased the two extra rims you had which were powder coated.  A few years later I purchased two more and chemically stripped and painted those myself with a rattle can`.  Total investment about $10 and two hours of my time.  When I bought new tires two months ago, I finally fitted the wheels to the car.  I have the painted ones up front and the powdered ones on the rear.  By the way, I had to sand the powder coating off of the inside of the hub hole because the powder coating was so thick it prevented the wheels from seating over the hub.   As they are all hidden under wheel covers, no one can see the surfaces anyway.  After a second tire rotation cycle, maybe in a year or two, I'll let you know if there is any difference. 

[dseretakis]

I agree. Powder coat may leave a nice finish but it is noticeably different from paint. If going for originality, and I think that's probably wise these days considering what correctly restored examples of our cars are fetching, I'd use paint.
Also, when powder coat fails, it fails miserably. I've seen it peel of in sheets leaving a rusty surface behind.

I question why anyone would want to powder coat any part when heat has to be used to cure. For wheels a good walnut shell blast followed up with a good primer and paint will duplicate (and may improve upon) what came from the the factory. Ten years ago I had a choice for my wheels and stayed with the aforementioned process. No problems whatsoever. They still look great.

The new catalytic paints cure extremely well and are very durable.

Now if I just keep the brake dust off the front wheels...

George
1965 230SL

[mdsalemi]

Powder coat, like properly done paint, can have a nice finish. It can be dead-flat matte, to super-wet glossy, and anything in between. It depends on what you want, and I'd defy anyone to tell the difference when sheen is matched.

Powder coat "peeling off in sheets" is indication of failure for sure...just like paint peeling off in sheets. Anyone see some GM cars of a few years back? Like paint it's all in the preparation quality and installation quality.

Raymond, the ORIGINAL finish on those aluminum wheels, BEFORE I had them RE-powder coated, was powder coat!  That's what they told me and that's what they did at the factory. It's a harder, more durable finish which is why its used. More power to you Ray--I've never had any spray paint from a can finish that I'd be proud of, even if its covered up. The rest of the world might not see it, but I surely know about it, and realistically to me, that's more important.

Nobody has to do powder coating of anything--it's purely up to the owner. But there's good and bad suppliers, just as sure as there is good and bad paint. Remember the original topic here--not about the merits of powder coat versus paint, but about someone (a member here) who had a bad powder coat job.