Archinect
anchor

Finish options for exterior steel

reminiscences82

Hello all,

I was a bit confused from the options available in the market for finishing solid exterior steel - columns, copings, gates etc not something with holes like - grates, fences and so on. I would really appreciate any insight into when you would go for either of these options (cost, performance, maintenance, ease of finishing not in any specific order of priority): 

High Performance Coating (HP- Coating) : seems a three-coat highest quality finish product
Epoxy Paint 
Powder - coating
Pre-finished: 
galvanized, chrome-plated, stainless
Clear sealers

Are there differences on the above on whether they should be shop-applied or field-applied?

Are there any good resources on this topic people might have come across? Reaching out to individual product-reps seems to result in them pushing a specific product rather than ever getting a true picture of how this all works. Any expert insights or a direction where I can find some good info would be much appreciated!


 
Sep 13, 20 4:47 pm
Non Sequitur

You really should know the difference between these categories if you're anywhere close to design, fabrication, or specifying metal products.  This is basic fundamental stuff.

Sep 13, 20 5:10 pm  · 
 · 
apscoradiales

There is all kinds of info on the internet about metal finishes.

Once you think you know the one you want, contact a supplier or a manufacturer and start picking their brain. There are so many different options depending on what you are trying to do and where that it will make your head spin.

To go through them all would require a book. If you want to by-pass some of that, pick an architect in your area and ask to speak to their spec writer. They could direct you in the general direction you want to go.

Sep 13, 20 6:06 pm  · 
 · 

I personally would go with the powder coating for small items - keep in mind that these can still rust. 


For larger exterior items I'd use epoxy paint.  


Please keep in mind this is only my recommendation and is based on where I practice and the availability of qualified painters.  

Sep 14, 20 10:15 am  · 
 · 

Epoxies chalk and fade too much with UV. Better to go for a urethane high performance coating, or at least a urethane top coat over epoxy.

 · 

Again - based on my area and my access to installers . . . .

To be fair the chalking and fading has more to do with the colors chosen and the exposure of steel being painted.  Even a high performance coating will chalk and fade in my area if it's a high chromatic color with uninterrupted southern exposure. 


Also I've never done an epoxy coating without a urethane top coat.  


 · 

Isn’t that what I said?

 · 

If you’re wanting brighter colors in a system resistant to fading, best to use an FEVE resin top coat

 · 
Non Sequitur

How much salt does your stuff have to live with each winter? We've tried the high-performance marine-super paint stuff but it failed drastically because the low-bid GC failed to do proper prep.

 · 

I wish I had stats to back this up, but I'd say easily 90% of the paint or coating failures I've seen have come down to improper prep.

 · 

It's all about the prep. Oh and what I was saying that regardless of the coating type any high chroma color will chalk and fade in the area where I practice (high desert)

1  · 

You should check out FEVE resin top coats for your next project. Much better color retention and fade resistance than urethanes or other resin types, especially with high chroma colors. 15 year warranties against color fade and gloss retention for some of the products I've looked at.

 · 

Cool! Thanks EA!

1  · 
reminiscences82

thanks guys!

 · 
reminiscences82

seems like an Epoxy base coat with FEVE top coat seems the way to go. Seems like higher quality cousin of PVDF stuff like Kynar 500, urethane coatings.

Exciting product! Thanks for nudging me in this direction!

 · 
natematt

EA, do you think it really has much of a performance add vs PVDF for an average color?

PVDF is really popular with companies that do coil products, and a lot of them will give you a 30 year warranty on chalking and fading. 

Obviously if it's a bright or glossy color that is different. (But they don't offer those colors anyway because they can't warranty them)

 · 

Quick answer is that FEVE systems are slightly lower performing than PVDF systems (depending on the formulation, etc.). But here is the big difference. PVDF resins are heat cured meaning you have to bake them in an oven. So they work well for coil coating and aluminum extrusions, but not for structural steel and that sort of thing (I have heard that there are some PVDF formulations that can be 'air-dried' but I haven't come across any that I know of). FEVE doesn't need to bake in an oven. So they can be site applied and still cure properly. 

So for things like structural steel, they are ideal when you're looking for high chroma colors (FEVE actually outperforms PVDF in this regard I believe), and color and fade resistance (FEVE slightly under performs PVDF, but outperforms urethane, epoxy, or acrylics). Hope that helps. Also keep in mind that for all you know I'm just an anonymous wanker on the internet. Best to reach out to your local high performance coating reps to help you for your actual work.

 · 
archanonymous

go stainless or go home.

Sep 14, 20 11:06 am  · 
2  · 

Clad everything in copper or STFU! ;)

1  · 
atelier nobody

Corten

Sep 14, 20 12:35 pm  · 
2  · 
SneakyPete

These days a lot of "Corten" is aluminum, I believe.

 · 
apscoradiales

You want a relatively durable paint job?

Do a hot-dipped zinc thingy, then take the steel to a quality car paint shop.

Watch on Youtube how Audi paints their cars, and imitate.

Sep 14, 20 2:31 pm  · 
 · 
natematt

Someone else chime in on this if you have thoughts, i'd be interested to hear. 

But I just wanted to add, that it's also probably a good idea to use galv steel under the finishes where you can no matter how much you like the product. Rust stains on a brand new building are a bad look.... 

Sep 15, 20 3:23 am  · 
 · 
Non Sequitur

We do galvanized first everything because, well, we’re buried under ice and road salt 6 months per year. If a paint finish is required for galvanized steel then the GC will sandblast prior to paint but it does impact the galvanized “coating”. We only do this for decorative steel. Anything structural remains as is from the hot dip bath or we spec stainless.

 · 

I've met my fair share of architects that avoid duplex systems (paint over galvanized) because the paint often doesn't adhere to galvanized steel very well. Well, that's not true if it is prepped correctly (see my earlier comment).

Duplex systems are a very good system for exterior steel, especially if the owner is not one to keep up and maintain a paint barrier. They're more costly initially though, so you need an owner that is willing to pay a little more upfront.

 · 
natematt

I've never heard it refereed to as a duplex system before.

I have almost exclusively done it whenever we paint steel. we've done a few where the clients were too cheap to do the galv, and it was bad.... Even in a climate where rust isn't as big of an issue (socal) 

It seems like anything that's not a self contained pre-finished product with minimal interface is going to have a strong chance of bleeding.... 

 · 

Not everything needs to be galvanized for most projects in normal climates (sorry NS), shop-applied zinc-rich primers are fine. The issues usually occur at areas where the primer was damaged and not touched up, or where it was not present (field welds, etc.) and not applied properly (preparation issues) before finishing. One of my favorite games is to find the rust spots at field welds on structural steel and railings. 

For the projects I work on, exterior steel is either using zinc-rich primer (and paint), or getting galvanized for cathodic protection (most of the time galvanized will be painted, but if it doesn't matter aesthetically, it could be left exposed). I've never relied on barrier protection alone. One team wanted less than that with a penetrating oil (no protective film) to protect bare exterior steel (mill scale as "blackened steel") and I basically laughed at them. They ended up with zinc-rich primer and black urethane paint. Another project the contractor tried to use the direct to metal acrylic specified for the interior on the exterior steel, and that didn't last long at all (I thought I had some photos, but I can't find them at the moment).

 · 

While I'm handing out free information on the internet, I'll point a few more things for the record. Powder-coating is just an application process. You can still get powders in many different resin types and they will have different performance characteristics that match those types.

Understand how you are protecting the steel from corrosion. Two major options are barrier protection and cathodic protection. Barrier protection relies on a perfect barrier (i.e. paint film) to prevent corroding elements from contacting the steel. Cathodic protection relies on zinc to be sacrificial and protect the steel from corrosion.

Some steel substrates in certain environmental conditions benefit more from a zinc-rich primer than they would galvanized steel, even though there is more zinc in a galvanized coating than there would be in a zinc-rich primer. The underlying thinking behind this is that the resin matrix that binds the zinc in a zinc-rich primer to the steel offers some barrier protection even if the zinc is corroded away. So if the conditions would more rapidly corrode away the zinc, there might be a case for using less zinc in a primer than you would get from using galvanized steel.

Finally, if you want more zinc on your steel when using a zinc-rich primer, you cannot just add another coat of the zinc-rich primer (no joke this was a serious proposal on a project). The primer bonds well to properly prepared steel, but does not bond well to itself. So while you may think you can double the amount of zinc protecting your steel by adding a second coat of primer, it actually creates a weak point that will probably end up causing your coating to fail. 

Sep 15, 20 1:29 pm  · 
2  · 
SneakyPete

Ahem.


1  · 

What do you expect from the guy creating threads like, Show us your caulk!

1  · 
reminiscences82

thank you so much for your insights!

- very cool to learn about the 'duplex' approach of both cathodic and barrier protection. I always had the presumption paint would peel off galvanized steel. Seems like the preparation of the galvanized surface is super labor intensive and would have higher up-front costs:

https://galvanizeit.org/specif...
I wonder how the payback of this might compare vs regularly maintaining coatings on non-galvanized steel.


 · 

There is a link for that at the bottom of the article I linked to for duplex systems.

1  · 

For the nerds like me out there. This write up by Tnemec on the Amazon spheres project hits all the high points: high humidity (interior conditions that might as well be exterior) conditions, corrosion protection, demanding UV exposure (the glass and low-e coating was actually selected to allow as much UV through for the plants as possible while reducing heat gain, but that's another write up), shop-applied zinc-rich primer, field welds properly prepared and touched up with zinc-rich primer, epoxy intermediate coat, fluoropolymer top coat for color and gloss retention in lieu of urethane ...

Sep 15, 20 3:27 pm  · 
1  · 

Block this user


Are you sure you want to block this user and hide all related comments throughout the site?

  • ×Search in: