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So I have a small section (about 1.5 ft) of non-insulated ductwork with some rust right above my HVAC blower/furnace in the basement.

The rust seems to be caused by the water that sweats on that section when I'm running the AC.

I bought some hvac insulation that I could put over it, but I wanted to clean off the rust with a wire brush before I do so. My question is: do I also need to use some sort of rust stop or rust cleaner before I do so? And, if so, what kind of chemical/product would be safe for the ductwork? I don't want caustic or strong chemicals to make it into the ventilation system spreading fumes around the house or damaging equipment.

Do you think using loctite rust neutralizer would be safe? https://dm.henkel-dam.com/is/content/henkel/TDS-1381192-US-Loctite-Extend-Rust-Neutralizer-Bottle-8-fl-oz-2018-04-11pdf

Additional Update: Henkel corporation returned my email and said:

We would not recommend that the Loctite rust neutralizer be used in an enclosed system, such as, HVAC duct work.

  • Have you considered replacing the small section ? If it continues to be wet it will be (almost) impossible to stop the rust from recurring. – blacksmith37 Sep 12 at 19:15
  • I think it only gets wet or sweats because of a lack of insulation around that part. – MonkeyBonkey Sep 16 at 13:30
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Minimal solution is sand and prime.

Vinegar should not be used--for one thing it is so fluid that it will get into places where you cannot remove it and will cause corrosion. Naval jelly has phosphoric acid as the active ingredient and so it also should not get into places where it cannot be removed. But you could use naval jelly.

The safest removal would be simply hand or light power sanding and then priming. You probably would not need a top coat since the duct work is protected from the elements, but you could possibly get useful extra protection from a top coat.

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    They have a naval jelly that is a pretty thick gel. Pretty much stays where you put it... FYI – JACK Sep 12 at 16:56
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I have used naval jelly for many years and have had great results. It does, however, have that small California warning about being linked to cancer.

Vinegar also works just about as good as the naval jelly. Brush or pour it on and wait ten minutes and wipe off the area. Good luck.

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    Naval jelly will work, the duct should be galvanized so I would shy away from vinegar. Most any metal paint, especially one with a high zinc base would work. I tried a rust neutralizer years back according to the directions and it did not work as well as cold galvanizing spray paint in my opinion.+ – Ed Beal Sep 12 at 14:26
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The problem is that insulation, while it will reduce condensation, it will also trap moisture and hold it there. That, with the existing rust, will rapidly corrode the panel entirely.

The only "save the panel" treatment I can see involves aggressive mechanical rust removal followed by competent anti-rust coatings. All of which will involve strong smelling industrial chemicals which will spread throughout the house (this is an air handler after all). For some people, such things are "cootie" euther due to abundancee of caution, or genuine chemical sensitivities. The latter is, unfortunately, a real thing and should not be discounted.

You can play around with naval jelly / rust converter, but I've never had any luck with it in wet locations; it always fails within a year. The only things I've ever found to work in a wet location is

  • Media blast to near-white metal (SSPC-PC10) followed by dust removal, solvent wipedown, and almost any competent metal primer.
  • Power wirebrushing to futility, dust removal, solvent wipedown, then Rustoleum 7769 Rusty Metal Primer, or cold galvanizing compound. (Which is basically zinc powder held together by paint resin and thinner; it's so heavy it needs extensive shaking, and you should buy it in quarts so you don't break the paint shaker, seriously).

Every bit of this stuff is "cootie", however the (part intended to evaporate) fumes aren't as bad as the State of California says, and clear after a day or two. The resin (which should stay on the roller/brush) is a different matter, especially for the 2-part exotica, so don't spray it.

  • I'm not sure what you mean by "cootie?" Are you saying you have moral objections to some chemical? I'm looking for a technical or scientific explanation... i.e. if you use this X chemical in a ventilation system it will destroy some delicate part in the system, or that you'll be creating some noxious chemical that will damage filters or lungs or what not. Are you saying you have some religious objections to using chemicals - that seems absurd. – MonkeyBonkey Sep 16 at 13:29
  • A lot of people have a zero tolerance policy toward any foul smelling "industrial" chemical. For most IMO, that is not based on science, and yes, is absurd. However some truly do have compromised immune systems or multiple chemical sensitivity (unfortunately, that is a real thing, one of several new epidemics). "Cootie" is a catch-all for all such cases, whether rational or not. The takeaway is, If you paint an air handler with smelly paint, your house will smell for awhile. People may have problems with that. – Harper - Reinstate Monica Sep 16 at 18:39

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