Painting galvanized metal


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Old 05-30-23, 10:25 AM
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Painting galvanized metal

So I learned something new...

We have a couple bollards at our congregation that were new about 4 yrs ago, and someone painted them safety yellow using what I assume was a DTM (direct to metal) oil based paint. But within about 1 year the paint was all peeling off in big chunks. It didn't take too much effort to scrape all the old paint off, the adhesion of any remaining paint was poor. This was due to the oily, slick surface (passivator) on the galvanizing. And turns out all alkyd oil based paints will eventually react with the zinc on the surface and peel.

The passivator must be removed by SSPC-SP7 (meaning, a certain level of brush off-blast cleaning).

But apparently you can also successfully paint galvanized that has sufficiently weathered. The test is to see if you can chemically etch the galvanized with a solution of Copper Sulfate, salt and water. If it does NOT turn greenish black, the passivator remains on the surface and it must be removed by blasting. (Or weathering)

If it weathers TOO much, the galvanized will get a white oxide which must also be removed to successfully paint the surface.

So I mixed up about 70 mL of Copper Sulfate, 70 mL of salt and dissolved it in 700 mL of hot water. Sprayed it on the bollards with a spray bottle (gloves and goggles) and within a few seconds it was turning greenish black. And the whole surface seemed to take the etch evenly. This was no doubt due to about 6 months of weathering since we stripped the old paint.

So the next step is to prime and paint, but this time I notice that Sherwin Williams recommends only water-based/ latex paints and primers. So we'll see how that holds up this time.

I was wondering marksr, what did you find to work well on new galvanized metal that you painted?
 
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Old 05-30-23, 10:53 AM
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Way back when when oil base was king we let it weather for a year and then painted with oil primer and finish. If we had to paint it new we'd wash it down with vinegar first.

Over the last 40 yrs I've had good results just using latex paint over clean new galvanized. I don't think I ever used latex on a bollard post but then I know it's been 20-30yrs since I painted one.
 
 

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