Powdercoating stainless steel Q&A's, Problems and Solutions

A. Painting stainless is purely cosmetic and won't be resistant to chipping and other adhesion problems. You'll need to CYA if you paint it by making the customer sign something saying that he understands and agrees with the terms. Reprofiling the surface is like taking away the reason the product is made from stainless. Don't do it without his permission unless you want to buy it all back some day.

A. Powder coating SS is more common than most would think, powder coating SS gives you many advantages over coating CRS or HRS or all of the pre plated materials out there. The main advantage is that corrosion is at a very small percentage, and SS if prepped correctly it will hold up to adhesion as well as many other destruct test.

DOD/AERO/SPACE

A. Hello

I tried to coat over stainless steel sheets using Polyester powder electrostatic spraying & baking.

The stainless steel was brushed finish

We etched the surface with ANTOX

The powder coating was successful.

If you have not been able to obtain Antox material try a sample of Anoprime from the UK

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Ed. note: Chemical names or descriptions of the types of etchants, please, readers -- not proprietary names!

A. Sir,
Methylene Chloride is a paint stripper not a solvent degreaser.
That is possibly the beginning of your woes?

A. Hi Audrey. William is deliberately being very gentle :-)

First, as he says, methylene chloride is not a proper degreaser. It sounds quite unusual to me to use it in a vapor degreaser, and I'm not sure whether it is even legal in your jurisdiction. Wiping with a better solvent is sometimes done in unusual situations, but it is not a proper pretreatment.

I would suggest alkaline cleaning or sandblasting, followed by a self-etching primer. Good luck.

Regards,

A. Hi Akshay. I don't think there's any problem in powder coating onto polished stainless steel which has been properly pretreated. But if you are talking about simply powder coating directly onto polished steel, I think I'd expect terrible adhesion problems.

Regards,

A. Akshay,
I don't think there's any problem per se, but keep in mind you are:

1. Paying extra for stainless steel and then covering it over, making your choice of alloy a moot point.

2. Paying extra for a polished surface which, if anything, probably reduces the adhesion of the powder.

A. It seems to me that this is an ill defined question.
The metal finishing industry in the western world can and has overcome very many challenges over the centuries.
We always come back to the points that:
(1) you must fully define the substrate and it's condition.
(2) you must assess what facilities are available to the artisan.
(3) you must quantify the performance factors required of the finished article.
In this instance it is highly unlikely that organic coating wet or powder is going to provide any mechanical properties, i.e., Adhesion, impact resistance or abrasion resistance.
On the other hand if a purely aesthetic appearance is the key performance criterion then that is achievable.

In other words fully define the problem and the desired outcome before you can hope for a result.
Hope this helps.
Regards,
Bill

A. Hi Alex. Sounds to me like the stainless wasn't passivated before powder coating. If iron from previously used sand contacts stainless steel it will rust. The powder coating probably protected it for a year and that's all.

Regards,

A. It also sounds like there are microcracks in the stainless. Look at the coating carefully. Good luck.

  Excellent point, Phil. Thanks!

Regards,

A. Hi Adam. You already recognize what the actual problem is: that you seek diametric opposites, a very smooth finish for looks and a very toothy finish for adhesion.

We have numerous threads about etching stainless steel, which can be found by using the search term "etch stainless" in the google custom search engine. ferric chloride ⇦ this on eBay or Amazon [affil links] is probably the most common and universal etch, but a silver bullet that can both etch and not etch doesn't sound possible. And even if it were, expecting really good adhesion without pretreatments which will obscure the shine, doesn't sound promising.

Your idea of chrome-look paint/powder reasonable to me.

Regards,

A. Hi Anwar. More words please :-)

Do you mean that you first passivate the parts, but then you send them through a pretreatment process? And the area which has now been pretreated, but does not get powder coated, corrodes? If so, what pretreatment sequence must they endure?

Regards,

A. Hi,
having read through this thread I think I can lend some important information on stainless steel. I ran a body shop specializing in North American highway coaches for 15 years. These are built of mostly of 3xx series stainless steel and the outside skins were stainless or aluminum into the 90's. Today one manufacturer still skins in stainless steel. Is there a reason to paint or powder stainless? you bet. The stainless gave the rust prevention on the inside of the panels and the paint gave the desired look on the outside. It was routine to leave bare stainless on the bottom luggage doors for durability but the sash panels between those and the windows were always painted with company colours.
Adhesion: You have to understand how stainless resists corrosion. Oxidization. The surface of stainless steel you see is not stainless steel, you are seeing through a semi transparent layer of oxidization a few atoms thick. This keeps the rusting process from starting by sealing the surface (this oxidization is not rust) It is very very hard and smooth and nothing I know of will stick well to it. The oxidization has to be removed or else paint will peel off easily. I will describe the process we used that gave us a 100% success rate and no warrantee returns (and yes we had some before learning this process)
We scuffed the panels with red Scotch-brite pads making sure every rivet was also completely scuffed. You can see a colour change when you have torn that layer of oxidization off, it is less grey and brighter, and yes this is hard work, but for us the only one that was nondestructive and could get all details. (sandblasting warps the 22 ga. panels).
The critical part no one has mentioned here is time. This layer starts to rebuild immediately. We set a 2-1/2 hour time limit from the beginning of prep to the time the painter was starting to apply a vinyl wash etch primer. An etch primer will not - NOT - break down oxidization one day or more old, it is very tough stuff. An etch primer will however check the oxidization in the last 2 or so hours.

Our process was as follows; the coach was moved into the paint booth then completely masked. Then (no later than lunch) 4 staff were pulled off what they were doing and put on to scuffing both sides at the same time. Each starting at a corner and moving to the center, a fifth man was added if the front or rear was to be painted. This seemed to be the right amount of area for everyone. If anyone was done too soon it meant they missed something and went back and did it again (which was rare after awhile). At the two hour mark they all tacked off the dust, by the time I did the final tack check the painter was in the booth, hooked up to his air and loaded. He was pretty much on the trigger as we walked out for a well-deserved coffee break. The race was over and the rest of the job was routine (unless someone sanded through the primer which needs immediate attention.)
For anyone in the business this is not convenient but charge and schedule accordingly, both stainless steel and aluminum are risker to coat than steel, but knowing how to tame the beast reduces your risk to near zero. If two hours seems extreme, it isn't: this self-repairing oxidization process is quick. Scuff in the afternoon and paint the next morning you are toast -- it will fail. To see this, scuff a scrap of stainless wait a few hours and then scuff one half of it. The colour will already be a little different.
I really hope this helps all you painters and powder coaters out there as I think it is the same problem appearing with both processes.