If you have been in the hood cleaning trade for any length of time, you have heard the phrase a thousand times: "clean to bare metal." For years, those three words were the standard every greaser was held to, the words inspectors quoted, and the words customers argued about when they did not want to pay for a proper job.
Then, quietly, NFPA 96 dropped them.
Here is the real history — where "bare metal" came from, why it was added in the first place, why it caused so many arguments, and why the Technical Committee finally took it out. This is pulled straight from the actual NFPA committee documents we have collected going back over 50 years.
Before "Bare Metal": The 1970s
The cleaning rule in NFPA 96 is older than most people realize. The 1973 Technical Committee Report shows Section 1131 (the cleaning section in the old numbering) reading:
"Hoods, grease removal devices, fans, ducts, and other appurtenances shall be cleaned at frequent intervals prior to surfaces becoming heavily contaminated with grease or oily sludge."
Sound familiar? That sentence is the ancestor of the rule that still exists today. But notice what is missing: there is no "bare metal" anywhere. Through the 1976, 1980, 1983, and 1987 revision cycles, the language stayed basically the same — cleaned at frequent intervals, no flammable solvents, protect the detection devices, put everything back in service.
And here is the detail that sets up the whole story. That same 1973 report includes a note that reads:
"Satisfactory cleaning results have been obtained with a powder compound consisting of one part calcium hydroxide and two parts calcium carbonate. This compound saponifies the grease or oily sludge, thus making it easier to remove and clean."
That is the powder. It started life as a legitimate cleaning aid — a chemical trick to turn grease into soap so it would release from the metal. Keep it in mind, because the powder is the entire reason "bare metal" exists.
1991 Edition: "Bare Metal" Is Born — to Fight a Scam
By the late 1980s, the powder had mutated. The Technical Committee Report from the 1990 Fall Meeting (the cycle that produced the 1991 edition) contains Proposal 96-97 (Log #34), submitted by Larry Stahl of Hardee's Food Systems — a restaurant chain that had figured out what was being done to their ducts. His substantiation says it all:
"The internal surfaces of ventilation systems are being partially cleaned and then sprayed with a Fire Rating Powder. They leave a grease layer on the duct to hold the powder."
Read that again. Crews were leaving the grease in the duct on purpose, spraying powder over it, and selling it as fire protection. The "fire rating powder" needed the grease layer just to stick.
The committee accepted the proposal in principle, and their action was to insert three words into 8-3.1: "cleaned to bare metal at frequent intervals." Their statement: "More clearly meets the intent of the submitter."
That is the origin. "Clean to bare metal" was never about shiny ducts for their own sake — it was written to make the powder scam impossible. If the duct has to be bare metal, you cannot leave a grease layer behind to hold powder.
1994 Edition: Banning the Powder Outright
Three years later, the committee went further. In the 1993 Fall cycle (producing the 1994 edition), the committee added a second sentence to 8-3.1:
"After cleaning to bare metal, the exhaust system shall not be coated with powder or other substance."
The committee statement is blunt: "Powder compounds and other substances have continued being used, sometimes not as a method to facilitate cleaning, but as a substitute for cleaning to bare metal." The original 1973 cleaning aid had become so "misused and abused" that the standard now banned coating a clean duct with it at all.
But the cracks in "bare metal" were already showing. A commenter in that same 1993 cycle pointed out that "there can rarely be a situation where 100 percent of the entire system is cleaned to 'bare metal,'" and that it was irrational to hold the vent cleaner responsible for inaccessible ductwork that someone else designed and installed badly.
1998 Edition: Defining the Undefinable
During the 1998 edition cycle, the committee kept the bare metal language but had to explain it. They added annex note A-8-3.1:
"Cleaning to bare metal does not mean to remove the paint from a painted surface of the exhaust system."
Think about that. The committee had to officially clarify that "bare metal" did not actually mean bare metal. When your standard needs an annex note telling people not to take the mandatory words literally, the words are not doing their job.
The 1998 cycle also added the requirement that the entire exhaust system be inspected by "a properly trained, qualified, and certified company or person(s)" on the now-familiar schedule based on cooking volume. (Fun fact: the seed of that schedule was planted back in the same 1990 cycle that created "bare metal," when Phil Ackland proposed mandatory 3-month cleanings for high-volume operations like charbroiling and wok cooking.)
2001 Edition: Bare Metal Moves to Chapter 11
The 2001 edition reorganized inspection and cleaning into Chapter 11. The language moved but did not change:
11.4.2 Hoods, grease removal devices, fans, ducts, and other appurtenances shall be cleaned to bare metal prior to surfaces becoming heavily contaminated with grease or oily sludge.
11.4.8 After the exhaust system is cleaned to bare metal, it shall not be coated with powder or other substance.
So heading into the 2004 revision cycle, "bare metal" was still the law of the land.
2004 Edition: The Committee Pulls the Plug
The change did not come from an outside comment or an industry lobby. It came from the Technical Committee on Venting Systems for Cooking Appliances itself. In the Report on Proposals for the 2004 edition, Committee Proposal 96-34 (Log #CP10) recommended rewriting 11.4.2 to read:
"Hoods, grease removal devices, fans, ducts, and other appurtenances shall be cleaned to remove combustible contaminants prior to surfaces becoming heavily contaminated with grease or oily sludge."
The annex note defining bare metal was deleted entirely. The committee's substantiation is worth reading word for word, because it explains everything:
"Existing wording (Bare metal) is restrictive and may not account for certain conditions, which exist such as coated surfaces, which aid in the release of grease during cleaning.
Bare metal can be discolored or stained which AHJ can interpret as not clean."
Every veteran of this trade has lived both of those problems. A stained duct that is perfectly clean gets red-tagged by an inspector who thinks discoloration equals grease. And a coated or factory-finished surface that sheds grease beautifully during cleaning technically "fails" a literal bare-metal reading.
The same cycle (Log #41) also deleted the words "to bare metal" from 11.4.8, so the powder rule now reads "after the exhaust system is cleaned" — no bare metal anywhere.
The committee vote was not close: 24 affirmative, 3 ballots not returned, zero negative.
What Replaced It: A Number You Can Measure
Here is the part a lot of people miss. The committee did not just delete "bare metal" and leave a hole. In the same 2004 cycle, they replaced the old annex note with something far better: an actual measurement.
The new Annex A.11.4.2 introduced the depth gauge comb method. Scrape the comb along the duct surface and read the grease depth:
- 50 microns (0.002 in.) — the target level of cleanliness after a cleaning
- 2,000 microns (0.078 in.) — time to clean the duct
- 3,175 microns (0.125 in.) in a fan housing — requires cleaning
The standard moved from an argument ("is that bare metal or a stain?") to a number you can put a gauge on. That change carried forward into the later editions — in the 2014 edition it lives at Section 11.6.2 with the same "remove combustible contaminants" language, and Annex A.11.6.2 still shows the depth gauge comb figure.
The Whole Arc in One Paragraph
A powder invented as a legitimate cleaning aid in the 1970s got twisted into a spray-over scam by the 1980s, so the committee wrote "clean to bare metal" into the 1991 edition to kill it. The phrase did its job against the powder but created a decade of enforcement arguments, so the committee deleted it in the 2004 edition and replaced a subjective phrase with measurable grease-depth numbers. The sentence that survives today — "cleaned to remove combustible contaminants prior to surfaces becoming heavily contaminated with grease or oily sludge" — still carries the DNA of Section 1131 from the 1973 report, word for word.
What This Means on the Job
- "Bare metal" has not been NFPA 96 code language since the 2001 edition. Anyone citing it as a current NFPA 96 requirement is working from a standard that is over two decades old.
- The current language is arguably stricter, not looser. "Remove combustible contaminants" plus a 0.002 in. cleanliness target is a measurable performance standard. A smear of grease on a shiny duct passes a "looks clean" eyeball test and fails the comb.
- Your AHJ still calls the shots. Local inspectors and fire marshals can and do enforce stricter local standards, and plenty of jurisdictions still say "bare metal" out of habit. Know what your local authority actually enforces.
- The paper trail matters more than ever. Because "clean" is now a measurable condition, before-and-after photos and documentation of what you cleaned (and what you could not access) are your best defense when an AHJ or a customer questions the work.
- If someone offers you a miracle powder or coating instead of a cleaning, run. The committee has been fighting that battle since 1990. It was a scam then and it is a scam now.
Why We Care
At Bryan Exhaust Tools, we are not just selling scrapers and foamers — we have been on the other end of this argument for 20+ years, running a real exhaust cleaning operation. We lived through the "bare metal" era, the powder-substitute era, and every vague-standards argument in between. The move from a subjective phrase to a measurable standard was good for professional cleaners who do the job right, because it rewards real cleaning over appearances.
Tools Built for Greasers, by Greasers Right Here In The USA 🇺🇸.
Source documents: NFPA 96 Technical Committee Report, 1973 (Section 1131 and powder note); NFPA 96 Technical Committee Report, Fall 1976 (Section 8-3.1); NFPA 96 Technical Committee Report, Fall 1990 (Proposal 96-97, Log #34; Proposal 96-98, Log #61 — 1991 edition cycle); NFPA 96 Technical Committee Report and Documentation, Fall 1993 (1994 edition cycle); NFPA 96 Report on Proposals and Report on Comments, Annual 1998 (1998 edition cycle); NFPA 96 Report on Proposals, May 2001 (2001 edition); NFPA 96 Standard, 2001 edition, Sections 11.3-11.4; NFPA 96 Report on Proposals, May 2004 (Committee Proposal 96-34, Log #CP10; Log #41); NFPA 96 Standard, 2014 edition, Section 11.6 and Annex A.11.6.2.
This article is for general information and education. NFPA 96 is a copyrighted standard — always refer to the current official edition and your local authority having jurisdiction for compliance.