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I have some fence and gate posts made of square steel tubing that were left open on the top. Rain happily collects down the holes so I drilled a weep hole to let the water that collects out. More than I expected was in there as I was sprayed with brown water.

Now it's frequently dribbling brown rusty water down after it rains. Better out than in?

Anyways, the posts (tubes) are 4-8 feet long and I'm not quite sure how to protect the inside. If I shoot a bunch of Rustoleum inside would that be enough? I was going to get some caps for the top to prevent most water from getting in, but don't know if the rust already in there will continue to be an issue. Urethane foam to fill it?

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    I'd think foam would simply hold moisture against the ID of the tube.
    – Huesmann
    Aug 19, 2023 at 21:54
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    You did the thing by drilling holes. Now put a cap. Shooting paint at rust doesn't really do anything and is debatably worse.
    – Mazura
    Aug 19, 2023 at 22:06
  • Wouldn't flowing water be worse than just sitting water? Flowing water carries the rust away, allowing more iron to oxidize. Sitting water will rust the surface, but the rust layer becomes a protective layer because rust itself isn't the issue. I think capping the top is enough to make it last much longer.
    – Nelson
    Aug 21, 2023 at 1:47

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I believe Rustoleum paint would be ineffective unless the surface could be cleaned of rust down to clean metal.

The product that comes to mind to clean off the rust is "Naval Jelly" or other phosphoric acid rust treatment. But it is not clear exactly how this would be applied to the inside of the posts embedded in the ground. If the posts are completely open at the bottom and filled at the bottom with soil or concrete, then it is not clear how one would remove the unreacted phosphoric acid. Leaving unreacted phosphoric acid inside the posts might cause corrosion over time.

The safest approach might be to dry out the interior of the pipes by pulling air through them with a shop vac on the weep holes. Once the interior is dry coat the interior with oil and put a cap on the top to keep out rain water.

Instead of simple oil you might use a product for stopping corrosion inside the tubing of steel framed bicycles (so called "frame saver"), but those bicycle products are rather expensive and this would probably be too costly for fence posts.

In the end the best approach might be to just cover the top of the posts and assume that the walls of the pipe are thick enough that the lifetime of the posts will be sufficiently long without having to take expensive protective measures.

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Fill with cement/concrete; slow .tedious. I used sand mix for cyclone fence posts , more to strengthen them than stop rust. I only lived in the house a few more years but never saw any rusty water.

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