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"Rusted sheet metal HVAC ducting"
Current question:September 4, 2021
Q. When I bought the house, I had noticed this rust-like stuff on the end of the HVAC duct work but I want to make sure that it's just rust and not mold.
So I'm curious what the yellow stuff is? How do I clean this? Or does it need to be removed and replaced? Thank you.Dylan Bruns
- Saint Louis, Missouri
A. Hi. The orange color looks a lot like rust to me, but rust doesn't make lumps of that sort. I'm not sure from the photos if I am looking at pebbles stuck on the sheet, perhaps from this sheet sitting on wet concrete before erection, or if those lumps are an orange colored mold.
You might take a look at
because in person you may be able to see something that I can't grasp from the photos.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Closely related Q&A's, oldest first:2005
Q. My home furnace is in a crawl space, & some of the sheet metal ducting has developed rust in spots in low-point areas of soil contact, unnoticed until recently. I have removed the offending soil, but need to know how best to treat the rusted areas. The rusted areas are close to the furnace and are visible and accessible when I remove the filter cover to replace the filter. Removal of the ducts is NOT within my personal capability, so I will have to do any work from inside----when the furnace is secured, of course. Any recommendations on how best to proceed?JOHN WALKER
HOMEOWNER - LEESBURG, Virginia
A. Use sandpaper or 0000 steel wool [affil. link to info/product at Rockler] to get the rust off (or a commercial solvent) paint with furnace/fireplace paint (available at most home improvement stores).
My above suggestion is based on the guess that the rust is due to overheating of spots on the ducts. If these same ducts also run your cooling, I would suggest using a different pain (but still sanding the rust off for first step on surface prep). Use an epoxy based paint if it is due to excessive moisture.
Put some thought into what could have made these rust spots, was it high moisture? Excessive heat? poor plating? the magical duct gnomes?
The more we know the more that we can suggest.Marc Banks
- Elizabeth City, North Carolina
Q. MARC BANKS,
THANKS FOR YOUR RESPONSE. THE DUCTS ALSO SERVE MY AC, & THE CAUSE OF THE RUST WAS MOST LIKELY MOISTURE FROM THE SOIL CONTACT, AS IT OCCURS ONLY IN THOSE AREAS. I CAN REACH MOST OF THE AREAS FROM INSIDE, AND WILL BE ABLE TO SAND THEM, BUT WILL NOT BE ABLE TO REACH ALL OF THE OUTSIDE AREAS. CAN YOU RECOMMEND A PARTICULAR COMMERCIAL RUST SOLVENT FOR USE (THE METAL LOOKS GALVANIZED TO ME)?JOHN WALKER
- LEESBURG, Virginia
A. There are a number of commercially available solvents for rust, it sounds like you are only dealing with surface rust so you aren't going to have to use a horribly powerful remover. I'm going to suggest something that Goran Budija has mentioned a few times (he posts on here now and again) "5% citric acid solution (or 50 gm citric acid/1 lit water/+ some ammonia, pH must be 3,5)" A much more environmentally friendly solution, you can get powdered citric acid at many natural food stores, if you have heavier rust you can make it a bit more powerful (double the amount of citric acid in the solution and you have a 10% solution)
Anywho, if you want an industrial cleaner stop by whatever hardware store is nearby, tell them you need a rust remover and see what they have (most of the time I need to derust something I end up using an aggressive acid mixture, I don't suggest it unless you have the cleanup gear for it).
As for the paint? Rustoleum has high-temp paint for outdoor barbecue grills, it's robust, waterproof, and stands up to reasonably high temperatures. And as another bonus, it's spray paint (for those hard to reach areas). Let me know if you need anything else I'll be lurking around here.Marc Banks
- Elizabeth City, North Carolina
April 16, 2015
Q. I also have rust in my ducts. They are in a slab floor. One of the ducts has a visible hole. I have heard of a plastic coating for this. What is it and how effective is it?P Madison
- BROKEN ARROW Oklahoma USA
May 6, 2015
A. P Madison,
Mastic is a sealing putty commonly used in the HVAC industry. If the holes are very small, just apply mastic and let it dry. If the holes are very large you could attach a fiberglass mesh across each hole and then float mastic over the mesh. You can also use mastic tape.
Feather Hollow Eng. - Stockton, California