Painting galvanized metal pipe and sanding

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Old 05-11-20, 01:04 PM
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Painting galvanized metal pipe and sanding

I know very little about metals so please bear with me.

I have a galvanized metal pipe that I wanted to spray paint. After seeing that regular spray paint can't be applied directly to the galvanized pipe, I decided to use an acrylic primer, and then spray paint over that as a top-coat.

The problem is that before I applied the primer, I followed some advice online to sand the pipe to roughen the surface. Well, I used 80-grit sandpaper, and after sanding, the surface is smoother than the original!

Despite that, I proceeded to apply the primer, and after it dried, I saw that there are tiny miniscule cracks in the paint everywhere. I then tried to scrape the paint a bit to see if it was sticking, and it scraped off fairly easily.

I tried to figure out what I did wrong, and also did some experiments on a bottom section of the pipe that was not sanded. I noticed:
  • There is more cracking where the primer was thicker. Maybe I applied the primer too thick?
  • The paint was harder to scrape off in the unsanded portion of the pipe.
Do you think the cracking and bonding issues are related?

If I messed up the surface by sanding it, what's the best way to correct it?

A separate but related question: after I sanded the pipe, the surface is now shiny (but with the streaks caused by the sanding). Does that mean that I sanded away the galvanized zinc layer? (I just used regular 80-grit sandpaper--is that possible)? And if so, does it matter since I'm painting over it anyway?
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Old 05-11-20, 02:02 PM
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What is the application with this pipe?

Hard to say without seeing but latex primers/paints will crack if applied to heavy or if the substrate is very hot. The solvents in some aerosol paints can 'melt' the underlying coating.
 
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Old 05-11-20, 02:23 PM
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If you are sanding a electro plated pipe like EMT then it is very easy to sand through the galvanized layer as it's quite thin and zinc is soft.

When you say you used an acrylic primer, what primer did you use specifically? Was it oil or water based?

Sanding isn't required but it can be done for better adhesion and a smoother surface. Don't forget to thoroughly clean the part with solvent before painting. For painting a steel pipe I'd use a spray can of primer intended for steel and then top coat with another spray paint that is compatible.
 
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Old 05-11-20, 02:45 PM
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What is the application with this pipe?
It's a post for holding a shade sail.

When you say you used an acrylic primer, what primer did you use specifically? Was it oil or water based?

It's the rust-oleum latex primer for aluminum and galvanized surfaces.


 
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Old 05-11-20, 02:53 PM
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I never heard that the coating is thin and can be sanded off, but you can use a green scotch pad to scuff and clean if it's a concern vs sanding!
 
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Old 05-11-20, 04:35 PM
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I have had poor luck with water based primers on metals. Oil based clean metal primer would be a better choice and they even list corrosion protection as one of it's features, something they don't with the water based.
 
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Old 05-12-20, 03:45 AM
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On light weight galvanized latex primer is generally best but with a heavy pipe like that I'd go oil base all the way.
 
 

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