I need to put up a sign. The post is a scrap piece of 3/4 inch galvanized plumbing pipe. On top will be the 12 x 6 inch cast steel sign. The sign will be 4 feet off the earth grade. I want to facilitate air movement in the pipe to keep the interior dry. I have drilled two 1/8 inch venting holes near the bottom to let air in. I have capped the pipe but drilled a 5/16 inch venting hole on top to allow air to escape.
This is probably good enough to keep the pipe interior dry. But now for the hypothetical.
For reasons beyond human comprehension I can only paint one half of the exposed 3/4 inch pipe black. The remainder will be unpainted. For maximum internal dryness, should I paint the upper or lower half of the pipe?
It would stay driest if you painted the south half black, assuming the pole will be in the northern hemisphere. And plug up at least the top hole so no rain (or other water dropping down) gets in, assuming it rains where it will reside.
Is there some particular reason to want to keep the inside of a galvanized water pipe dry? That sort of thing is generally pretty resistant to moisture. Is humidity inside the pipe expected to do more damage than the rain/snow/dew on the outside of the pipe?
Adding holes to it, whether at the top or bottom, just adds places for water to intrude. Is the pipe going to be set into the ground directly or into concrete? Why not cap both ends of the pipe and add sealant to the threads before setting it in place? Or pack the bottom end with silicone caulk or epoxy putty before putting it in the ground? Or fill the whole pipe with cement, or paraffin, or dip it in a sealant like Plasti-Dip or simple enamel paint?
I mean, capping both ends without holes anywhere would have to do a better job of keeping the interior dry unless there’s something else we’re missing. Do that, and the paint details won’t matter.
Dickerman
Yes, I live in the northern hemisphere. And your response was unexpected. You are correct that I could paint the south side. After all, it would be one half. But my OP did say upper or lower half.
brossa
Your points are well taken about galvanized pipe being water resistant. And this pipe will be encased in concrete and then put in the ground. I personally think, without a shred of factual evidence, that keeping the interior of a pipe dry is a sound thing to attempt. Yes, I could seal both ends of the pipe. But condensation is water – inside the pipe. Even if I installed the sealed assembly right now, in August, I am convinced that a sufficient amount of water will still infiltrate over time to be detrimental. Filling the pipe with anything is overkill for my particular application. But then, perhaps my interest in keeping the interior dry is as well.
Sigene
Okay, one vote for the upper half.
Tripolar
Disagree. I want a convection event. A pipe, painted or not, will get hot in the sun. I want physics to work for me. Black paint will measurably increase the temperature of the metal pipe.
Well, for sure water is going to get in now because of the holes that are already drilled in the pipe and cap. Corrosion will also start at the places where the galvanizing was breached, ie at the drill holes. So now you just want to get the pipe to dry out as much as possible between inevitable wettings by painting either the top half or the bottom half black? It’s apparently too late to just use an intact pipe with sealed ends…
In that case, paint the bottom half. That’s where the water is going to collect. Heating that part up more will cause more evaporation of any liquid water than heating the upper half will.
As @brossa says, drilling holes in the pipe, has introduced exactly what you didn’t want, corrosion points that will prematurely rust out your pipe. I will be less tactful that Brossa: by drilling holes, you basically screwed yourself.
A galvanized pipe’s steel is sandwiched between exterior and interior zinc layers. Since the pipe surface is fully enveloped on all sides there was absolutely no reason to drill air holes into it. (Note: The interior steel is so sensitive to corrosion, that when welded a zinc coating must be reapplied to that very small area where the welding removed the zinc.)
Drilling the holes in the sides has exposed the non-coated interior steel to water etc. That will absolutely speed the deterioration of your pipe, especially the end in the ground or cement. It doesn’t matter what you do now to increase airflow, even humidity in the air will prematurely rust out the pipe.
Your question should be: “Now that I’ve done this, what is the best option to salvage the situation?”
One vote here for getting a new pipe and starting again and cap on both ends.
(3/4" galvanized steel pipe is $1.75 / foot at Home Depot, so I don’t think there should be an issue with cost.)
I don’t think my two small holes near the bottom of the pipe will cause excessive rusting at the breach point. The upper hole certainly won’t because the hole is in the cap, not the pipe. I can replace the cap in 50 years.
One could make the argument that the exposed steel pipe threads at the top will fail long before the dry interior.
I’m overthinking this. Even a plain steel pipe with many holes would also last 50 years. I’ll skip painting altogether and let my grandchildren replace it.
Find a bucket/can as deep/tall as possible to hold the paint, then dip the pipe in it. That way, not only will some paint get on the inside as well, if the paint is thick enough it might even cover some of the holes you drilled. Plus, no need to clean any brushes! Don’t worry if you dip the wrong end in; in that case, simply install the pipe the other way up. You’re welcome!
What work do you expect physics to do for you? As pointed out above once you put holes in the pipe and break through the galvanized layer the corrosion will process from there. Once that starts the galvanizing layer will peel away in the area of corrosion and continue. Painting some portion of the pipe will not change anything. If you put up 10 poles like that in the same small area, painted and unpainted in different portions, and came back to check them in 10 year intervals you will not find any distinct relationship between the location of the paint and the degree of corrosion.
As you are being told, the way to make a post like that last is to cap both ends, and then paint the whole thing on the outside.
There will be more rusting at the drilled holes than there will be anywhere along the undamaged interior surface of the pipe. If scrap pipe is free, just use an undrilled piece with an intact cap and sleep easy.
Paint the lower half before planting it. Paint over the holes you drilled in the bottom will delay corrosion there. Corrosion at ground level seems to be the concern, roun’ here at least.