RCD Corporation #8 Mastic Sheet Metal Air Duct Sealant

Why I reached for this mastic

On a recent attic retrofit with leaky trunk lines and a handful of suspect takeoffs, I wanted a sealant that would survive temperature swings, stick to galvanized steel, and not fumigate the house. I pulled out a tub of RCD #8 mastic. It’s a grey, UL-listed sheet metal sealant that’s non-flammable, low odor, and made in the USA. I’ve used plenty of mastics and tapes over the years; this one sits in my “default” bucket for everyday duct sealing because it consistently balances ease of use with a durable, code-friendly seal.

What it is, and what it’s good at

This is a brush-on, trowel-on duct mastic for sealing joints and seams in metal HVAC ductwork. If you’ve ever fought with foil tape on dusty or irregular joints, mastic like this is the more forgiving alternative. It’s thick enough to bridge typical seams, grabs onto clean sheet metal, and cures into a tough, slightly flexible layer that resists cracking. The grey color helps visually verify coverage, which is handy during inspections.

A few traits matter in cramped mechanical spaces and attics:
- UL listed: That simplifies permit conversations and keeps things on the right side of codes.
- Non-flammable and low odor: I could work beside a running air handler without feeling like I needed a respirator break.
- Weather resistant: Exterior runs, attic heat, and crawlspace humidity haven’t fazed it for me.

Application: straightforward and forgiving

I approach mastic the same way I approach paint: prep is half the job. On this project, I wiped down the seams with a clean rag to knock off dust and any residual oil. The mastic benefits from a quick stir to even out consistency—nothing elaborate, just a couple of passes with a stir stick.

Tools: I used a cheap chip brush for most seams, a putty knife for wider joints, and a gloved hand for awkward corners. The consistency here is spot-on: thick enough that it doesn’t slump off vertical seams, yet spreadable so I don’t feel like I’m wrestling it. I prefer to butter both sides of a seam and then tool it into a continuous bead. For larger gaps (think questionable drive-cleat seams or a sloppy takeoff), I embedded a strip of fiberglass mesh into the first coat, then laid a second coat over it. That’s standard practice and gives the seam reinforcement that tape alone can’t match.

Cleanup was simple while wet—water and a rag took it off my tools and the occasional drip on the plenum. Once it sets, you’re scraping, so I keep a damp rag handy and work clean as I go.

Performance: airtight and reliable

The baseline test for any duct sealant is simple: does the air leak stop and stay stopped? After the first pass, I put the system under normal operation and checked suspect joints with a hand and a smoke pencil. The telltale wisps from corners and S-cleats were gone. I rechecked a week later after a few days of heating cycles and saw no hairline splits or pull-back at transitions.

What I particularly like about this mastic is that it cures into a resilient film that tolerates the predictable movement of sheet metal as it expands and contracts. I’ve had lesser products become chalky in hot attics and crumble after a season. This one has held up for me through heat and cold swings and the occasional exterior exposure under a weather collar. The weather-resistant claim has matched my experience: no softening in humid conditions, no brittleness in dry winter air.

Comfort and safety in use

Low odor isn’t just a nicety. In tight mechanical rooms, heavy solvent smells can be a deal-breaker, and non-flammable mastics make me more comfortable working near furnaces. With this mastic, I could seal returns in conditioned space without needing to vacate the area. That lowers the friction on service calls where time and occupant comfort matter.

Where it shines—and where it doesn’t

Strengths:
- Excellent adhesion to clean sheet metal, even on slightly irregular seams.
- Spreads easily without slumping, making vertical work manageable.
- Low odor and non-flammable characteristics make indoor work practical.
- UL listing keeps inspectors and energy raters at ease.
- Weather resistance expands its use beyond strictly interior runs.

Limitations:
- It’s still mastic—expect mess if you rush. Gloves and drop cloths save headaches.
- It’s not a gap filler for big voids; anything beyond a modest seam needs mesh or mechanical correction.
- Cure time is real. I avoid running high static pressure through freshly sealed joints until it’s had time to set up. Plan staging accordingly.
- Cold environments thicken the material. In winter, I keep the container warm so it’s brushable.
- The finished look is utilitarian grey. If you want a crisp, “showroom” duct aesthetic, this isn’t that.

Mastic versus tape (and when to combine)

Foil tape still has a place—especially for quick patches on clean, flat surfaces or where immediate handling is required. But on panned returns, uneven S-drive seams, or transitions where tape edges can lift, I trust mastic more. My standard approach on larger seams is a hybrid: a strip of UL-rated scrim tape to lightly bridge the joint, then mastic over the top. The tape provides structure; the mastic delivers airtight, long-term sealing. This mastic behaves well in that layered system—no lifting or fisheyes, and it wets the scrim evenly.

Application tips from the field

• Surface prep matters: Dust and oil are adhesion killers. A quick wipe pays dividends.
• Stir before use: Even a short mix evens consistency.
• Use mesh on anything wider than a hairline: It prevents future cracking.
• Go for continuous coverage: Feather the edges and avoid pinholes; two thinner coats beat one lumpy one.
• Keep a damp rag handy: Wipe drips immediately; don’t plan on cleaning dried mastic easily.
• Stage your work: Seal, then move to another section while the first sets. Come back for a light second coat over problem areas.
• Mind temperature: If the product feels like taffy, warm the container slightly so it spreads properly.

Packaging and handling

The grey color is practical—it’s easy to see what’s coated and what you missed, especially in dim attics. The container reseals well if you keep the rim clean; I press a layer of plastic film over the surface before closing the lid to minimize skinning between sessions. Expect a single unit to go farther than you think, especially if you’re not trying to bury every seam under a thick layer. Target what leaks, reinforce the bad joints, and keep things tidy.

Final thoughts and recommendation

RCD #8 mastic does what a good duct sealant should do: it goes on easily, sticks where it counts, cures into a durable film, and stays put through weather and seasonal cycling. The UL listing, low odor, and non-flammable formulation make it a straightforward choice for both residential and light commercial work. It isn’t magic—poor prep, oversized gaps, or rushed curing will undermine results—but used correctly, it delivers an airtight, long-lasting seal that outperforms tape on most irregular joints.

I recommend this mastic for HVAC pros and serious DIYers who want a reliable, code-friendly way to seal sheet metal ducts. It’s easy to apply, comfortable to use indoors, and resilient enough for attics, crawlspaces, and even exterior runs under proper protection. If you’re sealing ductwork and want to stop chasing lifted tape seams season after season, this is the bucket I’d keep within reach.