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I have some ductwork has been getting rusty because of condensation and I want to seal it with duct sealant and thin insulation on the parts that sweat/wet. Do I have to sand the existing rust off or can I just seal over it and stop the rust? Will it continue to rust more under the sealant?

It's difficult spot to get to with sanding and I'll have to open up the wall and cut through pipes if I want to get the angle for sanding, whereas I can probably get the duct sealant liquid and thread the insulation on to the spot using a thin brush, wires and gravity.

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So it's a much smaller job if I don't do the sanding.

MonkeyBonkey
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  • Use a rust-converting primer. – Hot Licks Dec 16 '22 at 19:11
  • You don't _need_ to clean it up, but it's got no motivation to stop rusting on its own. – FreeMan Dec 16 '22 at 19:15
  • if you just cover the rust, it will keep rusting – Ruskes Dec 16 '22 at 19:41
  • There have been several suggestions given, be sure to read and follow the instructions with each then make your choice. – Gil Dec 17 '22 at 02:07
  • Sealing over it will not stop the rust. Is replacing the ductwork a reasonable option? You might not have cut through any pipes that way. – RockPaperLz- Mask it or Casket Dec 17 '22 at 03:27
  • I'd be less worried about the rust and more if the paint is for galvanized or not. But you're going to use duct sealant. Sounds like "find out". Picture? If it's getting rusty because of condensation, then coating it, and especially with some insulation, then it will stop rusting (presuming, you're talking about the outside, but if inside is the problem and you seal it there, then also yes). What kind of insulation, self adhesive? Then you may want to do some actual surface prep. Picture. – Mazura Dec 17 '22 at 04:14
  • https://diy.stackexchange.com/a/90474/23295 "For removing rust, I decide how serious I am. Either I'm willing to break out the gear and do full-on media blast to SSPC.SP10... or I'm not. If I'm not, then I do what I can with hand wirebrushing and palm sanders, heat it over 212F with a hair dryer (to remove all water), wipedown, and then..." - "go to the hardware store, buy whatever, paint it down and watch it fail in a few years. What you do at that point depends on what you used." – Mazura Dec 17 '22 at 04:24

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Use a rust converter. Be careful to get the proper product that chemically alters rust to make it non oxidizing, and not just a metal primer that covers rust. Example is here: Rust Reformer

Credit to Hot Licks for the original suggestion.

RockPaperLz- Mask it or Casket
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RMDman
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  • Have you had good experiences with rust converters? I've used quite a few of them, and although they converted the very top level of surface rust, the underlying rust always broke through that converted layer *very* quickly. – RockPaperLz- Mask it or Casket Dec 17 '22 at 03:29
  • I have used rust converter many times. Rarely did I get any rust continuing. It is a learning process. I discovered you need to cover the rust completely, soaking it so the product is actually absorbed into the rust. This gives better results than just spraying or brushing over the rust. – RMDman Dec 17 '22 at 13:44
  • Interesting. How did you soak it and for how long? I typically sprayed it or applied it with a brush. – RockPaperLz- Mask it or Casket Dec 18 '22 at 01:35
  • I like using the brush on stuff. Buy it from an automotive supplier. I just get plenty on the brush and slop it on the rust. I would remove any hard flaking rust, and protect the areas I didn't want it. The product states that rust must be present to be effective. – RMDman Dec 18 '22 at 02:13
  • That's what I've done, but had different results. Perhaps different brands, different compositions of metals, different thicknesses, different environmental conditions, etc. account for our different experiences. When I originally discovered the stuff, I thought *"this is too good to be true"*, and after using it, that was my conclusion. But after reading your experience (and assuming you don't profit from the stuff!), it's good to learn that at least someone had better experiences. – RockPaperLz- Mask it or Casket Dec 18 '22 at 06:05
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I found an article on your question in case you haven't seen it yet: https://azrust.com/clear-coat-over-rust/ In my personal experience, if rusted badly enough, the paint or sealer will flake off if the rust is flaking off (as you probably know). If this were the case, I would suggest going at it with a scraper, at the very least. You probably don't want to cut through the wall, but you should ask yourself if you don't do it now, is it going to be more work later? Although if the ductwork is bad enough, replace it, as 'RockPaperLz- Mask it or Casket' suggested.

Deyja S
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