Rusted sheet metal HVAC ducting

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
https://www.bustmold.com/resources/about-mold/types-of-mold/orange-mold/
because in person you may be able to see something that I can't grasp from the photos.

Luck & Regards,

A. Use sandpaper or 00 steel wool [on eBay or Amazon] 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.

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 [on eBay or Amazon] solution (or 50 gm citric acid /1 lit water/+ some ammonia [on eBay or Amazon], 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.

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.