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Subpart GG or 6H: Which Air Toxics Rule Governs Your Aerospace Coating Line
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Subpart GG or 6H: Which Air Toxics Rule Governs Your Aerospace Coating Line

Subpart GG applies only to aerospace facilities that are major sources of HAP. Below 10 and 25 tons per year, Subpart 6H governs instead. Here is how to run the test.

by iSi Environmental

In June 2006 EPA issued a final order against a helicopter plant in Stratford, Connecticut. Among the violations: the facility kept spraying primer and topcoat while the waterwalls controlling the booth exhaust lacked proper water pressure. The order cited 40 CFR part 63 subpart GG, the air toxics rule for aerospace manufacturing and rework, and the penalty came to $176,000 against a proposed $325,348 (ECHO case 01-2006-1005, docket CAA-01-2006-0063).

The chemistry of the coating was not the problem. A booth parameter went out of range and nobody shut the line down.

That case is a useful entry point, because the first question an aerospace manufacturer or MRO asks when it adds a coating line or a hangar is not what the limits are. It is which rule applies. Two air toxics standards reach spray coating of aircraft parts, and only one of them can apply to any given facility. Getting that wrong means building a compliance program for the wrong regulation.

Does Subpart GG apply to my aerospace coating line, or does 6H?

40 CFR 63.741(a) conditions Subpart GG on two things at once: aerospace manufacture or rework, and major source status under 40 CFR 63.2. An aerospace coating operation at an area source is not a GG source. Area-source spray coating of metal or plastic parts falls under Subpart HHHHHH instead.

Read 63.741(a) closely. It says the subpart applies to facilities engaged in part or in whole in the manufacture or rework of commercial, civil or military aerospace vehicles or components “and that are major sources as defined in 40 CFR 63.2.” The aerospace work alone does not pull you in. The size does.

Subpart HHHHHH, which practitioners call 6H, runs the other way. 40 CFR 63.11170(b) defines its reach as area sources only, and 63.11169(d)(6) excludes anything already covered under another area source NESHAP. The two rules do not overlap. They partition.

What makes an aerospace facility a major source of hazardous air pollutants?

40 CFR 63.2 sets it at potential to emit, in the aggregate, 10 tons per year or more of any single hazardous air pollutant, or 25 tons per year or more of any combination. Potential to emit means maximum design capacity, with physical and operational limits counted only when they are enforceable.

Two details in that definition decide most real cases.

The first is aggregation. The test runs on the whole stationary source or group of stationary sources within a contiguous area and under common control. One booth does not get its own number. Primer, topcoat, specialty coatings, chemical milling maskants, hand-wipe solvents, flush cleaning, spray gun cleaning and depainting strippers all go into the same total.

The second is that it is potential, not actual. The same section defines potential to emit as the maximum capacity to emit under physical and operational design, and treats an operational limitation as part of that design only when the limitation or its effect on emissions is enforceable. A facility emitting 4 tons of a single HAP on current order volume, with the booth capacity to emit 14 if it ran full, is a major source unless something enforceable caps it.

Subpart 6H restates the same arithmetic in metric at 63.11170(b): 9.07 Mg (10 tons) for any single HAP, 22.68 Mg (25 tons) for any combination. Same line, two rules, opposite sides.

What are the HAP and VOC content limits for aerospace primers under Subpart GG?

40 CFR 63.745(c)(1) and (c)(2) set three primer figures: 350 g/L (2.9 lb/gal) in the general case, 540 g/L (4.5 lb/gal) for general aviation rework facilities, and 650 g/L (5.4 lb/gal) for exterior primer on large commercial aircraft at existing sources producing fully assembled large commercial aircraft.

The precise wording of the third figure matters, because it is narrow. It covers exterior primer applied to large commercial aircraft components, or to fully assembled large commercial aircraft, at existing affected sources that produce fully assembled large commercial aircraft. A supplier making large commercial aircraft parts does not get 650 g/L unless the source itself produces fully assembled large commercial aircraft.

Paragraph (c)(1) states these as organic HAP limits, in grams of organic HAP per liter of primer less water, as applied. Paragraph (c)(2) states the same three numbers as VOC limits, measured less water and exempt solvents. Compliance may be shown on either the HAP content or the VOC content, not both, with one exception: a primer containing HAP solvents that are exempt from the VOC definition must use the HAP content (63.749(d)(3)).

What is the topcoat limit under Subpart GG, and does it differ for rework?

40 CFR 63.745(c)(3) and (c)(4) set topcoat at 420 g/L (3.5 lb/gal) of coating less water, as applied, and 540 g/L (4.5 lb/gal) for general aviation rework facilities. Self-priming topcoats carry the same two figures. VOC limits are the same numbers, measured less water and exempt solvents.

Classification drives which number you are held to, and 40 CFR 63.742 draws the lines. Primer is the first layer and any subsequent identically formulated layers applied to the substrate. Topcoat is a coating applied over a primer for appearance, identification, camouflage or protection. Self-priming topcoat goes directly onto an uncoated vehicle or component. Specialty coatings are excluded from both definitions and carry their own limits in appendix A.

A coating filed in the wrong category moves its applicable limit by as much as 230 g/L. That is the kind of error that shows up in a semiannual report two years later.

When does Subpart GG require a spray booth to shut down?

40 CFR 63.745(g)(3) requires immediate shutdown when the dry filter pressure drop, or the waterwash water flow rate, falls outside the limits set by the filter or booth manufacturer or by locally prepared procedures. Operation does not resume until the parameter is back in range. EPA has assessed penalties on exactly this provision.

The duty has a monitoring predicate. 63.745(g)(2)(iv) requires a differential pressure gauge installed across the filter banks, with the pressure drop read and recorded once per shift, or an interlock system that automatically shuts the spray application system down when the pressure drop leaves the manufacturer’s recommended range. Waterwash systems get the same treatment on water flow rate under (g)(2)(v). Pumpless systems monitor whatever parameter the manufacturer names as the performance indicator.

So the rule gives a facility a choice: log a number every shift, or wire in an interlock and let the booth stop itself. The Stratford case is what the first option looks like when the logging does not drive a response.

What reports does a Subpart GG facility file, and do they go through CEDRI?

40 CFR 63.753 requires semiannual reports every six months from the notification of compliance status and annual reports beginning twelve months after. Only performance test results go to CEDRI, within 60 days of each test, under 63.753(f). GG’s routine periodic reports are not CEDRI submissions. Subpart 6H is the opposite.

This trips people who learned electronic reporting through 6H. Under 63.11175(c) and 63.11176(c), on and after May 9, 2023, a 6H source submits its initial notification, its notification of compliance status, its annual notification of changes report and its methylene chloride report to EPA through CEDRI via the Central Data Exchange. That change is the headline of the 2022 amendments, and iSi covered it in detail in EPA’s changes to the 6H NESHAP for paint stripping and surface coating.

GG never got the same treatment. Its semiannual and annual reports still go to the Administrator or the delegated state agency the old way. The only electronic channel GG specifies is 63.753(f), for performance test results collected with test methods supported by EPA’s Electronic Reporting Tool.

What goes into the GG semiannual report is also worth naming, because the content list reads like a list of things a facility forgets to track. Any instance where a noncompliant cleaning solvent was used for non-exempt hand-wipe cleaning. Any new hand-wipe solvent used in the prior six months with its composite vapor pressure. Any instance where a noncompliant spray gun cleaning method was used. Any leaking enclosed spray gun cleaner left unrepaired and in use more than 15 days. Every coating whose HAP or VOC content exceeded its limit. And every time a primer or topcoat operation was not immediately shut down when a filter pressure drop or waterwash flow rate went out of range.

Do I need a new notification when I add a spray booth or a hangar?

40 CFR 63.743(a)(10) splits it on inorganic HAP. A booth or hangar without potential to emit 10 tons per year of an individual inorganic HAP or 25 tons per year combined needs only an annual notification by March 1. Above either figure, advance notification and written EPA approval apply.

There is a second trap in the same scenario. Under 63.749(a)(2), an existing primer, topcoat, specialty coating or depainting operation that constructs or reconstructs a spray booth or hangar must comply with the new source inorganic HAP requirements for that booth upon startup. The rest of the facility stays on the existing source requirements.

The difference is not cosmetic. Existing-source dry particulate filters certify to Tables 2 and 3 of 63.745: greater than 90 percent efficiency at particles larger than 5.7 micrometers on the liquid phase challenge, and larger than 8.1 micrometers on the solid phase challenge. New-source filters certify to Tables 4 and 5: greater than 95 percent at 2.0 micrometers liquid and 2.5 micrometers solid, with additional data points down to 0.42 micrometers. That is a three-stage arrestor specification instead of a two-stage one, and it has to be in the booth specification before the booth is bought.

Can waterborne coatings or low coating volumes get me out of the GG coating limits?

Two exits exist. 40 CFR 63.741(i) relieves waterborne coatings whose manufacturer data show HAP and VOC at or below the limits for that coating type, with manufacturer data and annual purchase records kept five years. 40 CFR 63.741(g) exempts low-volume coatings under 189 L (50 gal) per formulation and 757 L (200 gal) combined annually.

Both are paperwork-dependent. The waterborne exemption runs on manufacturer-supplied data that the facility has to hold and produce, and the records have to stay available for five years. The low-volume exemption runs on annual usage totals per formulation, which means purchase and usage records good enough to defend the 50-gallon and 200-gallon figures at an inspection.

Two more exits are worth knowing. 63.741(j) exempts rework of antique aerospace vehicles and components from the subpart entirely. And 63.746(a) says the depainting standards do not apply to a facility that depaints six or fewer completed aerospace vehicles in a calendar year, which keeps a lot of small MRO operations out of the depainting provisions while leaving the coating provisions intact.

What does Subpart 6H require of an area-source aerospace coating operation?

40 CFR 63.11173(e) requires certified painters, all spray-applied coatings in a booth or enclosure with filters demonstrated to 98 percent overspray capture per ANSI/ASHRAE 52.2-2017, HVLP or equivalent spray guns, and non-atomizing gun cleaning. The existing-source compliance date was January 10, 2011. No coating content limit applies.

The target HAP list is what pulls an aerospace supplier into 6H in the first place: compounds of chromium, lead, manganese, nickel or cadmium, spray-applied to a part or product made of metal, plastic or a combination (63.11169(c)). Chromated primers put most aerospace coating work squarely inside that list.

Painter certification has a cadence worth calendaring. Under 63.11173(g), personnel must be trained and certified no later than 180 days after hiring, training is valid for no more than five years, and everyone gets refresher training and re-certification every five years. An employee who transfers into a painter role is treated as a new hire.

One structural relief: 63.11174(b) exempts a 6H area source from the obligation to obtain a part 70 or 71 operating permit, provided nothing else requires one. A GG source, by definition, is a major source and has the Title V obligation that comes with it.

Does 6H’s HVLP spray gun requirement apply to aerospace work?

40 CFR 63.11173(e)(3) carves out three aerospace cases: components that normally require an airbrush or a spray gun extension to reach limited access spaces, coatings with fillers that adversely affect atomization with HVLP guns, and coatings with a dried film thickness normally under 0.0013 centimeter (0.0005 inch). The booth and training rules still apply.

This is the one place where 6H’s drafters wrote aerospace work into the rule by name, and it is narrow relief. It releases the spray gun technology requirement for specific applications. It does not release the enclosure, the 98 percent filter, the certified painter, the gun cleaning method, the notification or the recordkeeping.

6H also lists activities it does not reach at all, at 63.11169(d): surface coating or paint stripping performed on site at installations owned or operated by the Armed Forces, NASA or the National Nuclear Security Administration; military munitions and the equipment used exclusively to transport them; research and laboratory activities; quality control activities; and anything covered under another area source NESHAP. Space vehicle coating is separately excluded from the miscellaneous-parts trigger at 63.11170(a)(3).

Can an aerospace facility avoid Subpart GG by taking a synthetic minor HAP limit?

40 CFR 63.2 treats an operational limitation as part of a source’s design only when the limitation or its emissions effect is enforceable, and names title V permit terms limiting potential to emit as federally enforceable. In Kansas the vehicle is a Class II operating permit under K.A.R. 28-19-500(b). The cost is a permanent cap.

That is as far as the primary sources carry it, and it is worth stating carefully. The definition of potential to emit at 63.2 is what makes a cap work: a physical or operational limitation counts toward the design capacity when the limitation or its effect on emissions is enforceable. The definition of “federally enforceable” in the same section lists, among other things, “all terms and conditions in a title V permit, including any provisions that limit a source’s potential to emit, unless expressly designated as not federally enforceable.”

So a facility that holds a permit limiting HAP potential to emit below 10 and 25 tons per year is an area source, and 63.741(a) does not reach it. What the cap buys and what it costs are both real. KDHE’s own program summary describes Class II permits as the instrument for reducing potential to emit through federally enforceable limits (KDHE, Overview of the Kansas Air Quality Program). What it costs is a production ceiling you cannot exceed without reopening the permit, plus the usage tracking and reporting to prove you stayed under it.

For a growing supplier, the arithmetic on that trade is now available in EPA’s own numbers. See the burden comparison below.

Is Kansas delegated to implement Subpart GG, and what does KDHE require?

No part 63 delegation table for Kansas appears in 40 CFR 63.99(a), verified by full-text scan on 2026-10-13. K.A.R. 28-19-750 adopts 40 CFR part 63 as in effect July 1, 2010. Separately, K.A.R. 28-19-300(a) triggers a Kansas air construction permit at the same 10 and 25 tpy HAP figures.

For a Wichita facility that produces a convenient result. The calculation that decides whether Subpart GG applies is the same calculation that decides whether KDHE requires an air construction permit before the booth goes in: potential to emit 10 tons per year of a single HAP, or 25 tons per year combined. K.A.R. 28-19-500(a) sets the Class I operating permit, which satisfies Title V, at the same two HAP figures, and separately requires one for non-deferred NESHAP sources. Run the number once, use it for both filings.

The neighboring states diverge, and the divergence is in the CFR.

Oklahoma is delegated. The ODEQ table at 40 CFR 63.99(a)(37)(i) marks both Subpart GG and Subpart HHHHHH as delegated unchanged, with amendments after June 30, 2023 not delegated. EPA updated that delegation at 91 FR 52, effective February 2, 2026. For a Tulsa MRO line, ODEQ implements and enforces alongside EPA.

Texas is delegated. The TCEQ table at 40 CFR 63.99(a) marks both subparts delegated, with amendments after August 3, 2016 not delegated.

Missouri has no delegation table in 63.99, but its own rule is specific. 10 CSR 10-6.075 incorporates 40 CFR part 63 as promulgated July 1, 2019, and its subpart table lists Subpart GG with MoDNR as the primary regulating agency and Subpart HHHHHH with EPA as the primary. For a St. Louis aerospace plant that is the operative split: the state runs the aerospace rule, EPA runs the area source rule.

Colorado has no part 63 delegation table in 63.99 either. EPA Region 8 maintains its delegation summary outside the CFR, and the current status of Subpart GG for CDPHE could not be confirmed from a primary document. A Denver-area facility should ask CDPHE’s Air Pollution Control Division and EPA Region 8 directly rather than assume.

How much does Subpart GG compliance actually cost per facility?

EPA’s own paperwork estimates put Subpart GG at 144 respondents, 154,000 hours and $16,800,000 per year (89 FR 93580), against Subpart 6H’s 7,962 respondents, 101,972 hours and $12,100,000 (90 FR 45207). That is about 1,069 hours and $116,667 per GG facility per year, against about 13 hours and $1,520 per 6H facility.

The ratio on cost is roughly 77 to 1. That is the price of crossing 10 tons per year of a single HAP, measured in federal reporting burden alone, before any capital spend on a three-stage arrestor or an interlock.

Two things fall out of those numbers that a plant manager should weigh.

The GG universe is small and shrinking. EPA’s own aerospace NESHAP background material described the rule as reaching “at least 2,869 major source aerospace manufacturing and rework facilities” (EPA, Aerospace Manufacturing and Rework Facilities NESHAP). The 2024 paperwork renewal puts the current respondent count at 144. Whatever happened between those two figures, GG today reaches about 5 percent of the facility count in the rule’s own background documents.

And EPA says nothing is coming. In the same 2024 notice, explaining why the burden estimate did not move, EPA wrote that “the regulations have not changed over the past three years and are not anticipated to change over the next three years,” and that “the growth rate for this industry is very low or non-existent.” A Federal Register search on 2026-10-13 for aerospace manufacturing and rework, Subpart HHHHHH, and paint stripping and miscellaneous surface coating, from January 2024 forward, turns up only state delegation actions, SIP approvals and paperwork notices. No proposed rule. No reconsideration. No risk and technology review.

Where EPA enforcement has actually landed

Three Subpart GG cases sit in EPA’s ECHO enforcement database, and the pattern across them is consistent.

The Stratford helicopter plant, 2006, $176,000: a booth operating with the waterwalls short of water pressure, which is 63.745(g)(3) enforced on its face.

Naval Air Station Jacksonville, 2012, $52,739: the Navy depainted aircraft wing brackets subject to Subpart GG in a booth that had never been designated a MACT unit under GG or under the facility’s Title V permit. EPA cited 63.752(e)(4) for not maintaining the list of parts normally removed before depainting, 63.753(d) for missing the annual and semiannual depainting reports, and 63.6(e)(3) for having no startup, shutdown and malfunction plan for the booth (ECHO case 04-2011-1527, docket CAA-04-2011-157(b)).

Cessna Aircraft Company, Wichita, 2002, no penalty: EPA Region 7 issued an administrative compliance order requiring Cessna to comply with 63.744(a)(2) at every Cessna facility in the region, storing fresh and spent aerospace cleaning solvents in closed containers within 180 days and notifying EPA in writing (ECHO case 07-2003-0024, docket CAA-07-2003-0035).

A waterwall running dry. A booth missing from the permit. Open solvent cans. The cited provisions are 63.745(g)(3), 63.752, 63.753 and 63.744(a)(2), and not one of them is a grams-per-liter limit.

There is a counter-signal in the same data, and it deserves saying plainly. A query of all 1,190 ECHO enforcement cases against NAICS 3364 facilities, including the full case report for every one of the 225 Clean Air Act cases filed or settled from 2013 forward, turned up no case citing Subpart GG or any of its sections. The most recent GG-citing federal case in the database is the 2012 Navy consent agreement. Federal attention has been elsewhere for more than a decade.

That is not an argument for ignoring the rule. A GG source is a Title V source, and a deviation disclosed in a semiannual compliance certification creates its own exposure regardless of whether EPA is running an aerospace initiative. 40 CFR 19.4 currently sets the Clean Air Act judicial civil penalty at $124,426 per day for each violation, the administrative penalty at $59,114 per day with a $472,901 cap, and the field citation at $11,823 per day. Those figures were last adjusted by 90 FR 1377 in January 2025 and have not moved since.

The worker side of the same primer

The chromated primers that drive the inorganic HAP requirements in 63.745(g) are also what put an aerospace paint shop under 29 CFR 1910.1026. The hexavalent chromium permissible exposure limit is 5 micrograms per cubic meter as an 8-hour TWA, and the action level is 2.5. Aerospace work has its own provision at 1910.1026(f)(1)(ii), which requires engineering and work practice controls down to 25 micrograms per cubic meter for painting of aircraft or large aircraft parts, with respirators used to reach the PEL from there. That 25 figure is not a relaxed PEL for the aircraft industry, and iSi walked through why in hexavalent chromium exposure limits for manufacturing.

Running those two programs off one set of facts is the efficient path. The coating inventory that establishes your GG coating classifications is the same inventory that identifies which operations need a Cr(VI) exposure determination. The booth that satisfies 63.745(g)(1) with downward or crossdraft airflow is the same engineering control you will point to under 1910.1026(f).

What to do before the booth is ordered

The sequence that keeps a project out of trouble is short, and it runs front to back.

Compute facility-wide HAP potential to emit first, by pollutant, with the design basis written down for each source. That one document decides whether the next twenty obligations exist.

Decide deliberately whether you want to be a major source. If the answer is no, the enforceable cap has to be in the permit before the emissions capacity is in the building.

If you will be a GG source, specify the booth to the new-source filter tables, because a new booth at an existing facility is held to them on startup. Put the differential pressure gauge and the interlock decision in the equipment spec, not in a later retrofit.

Classify every coating against 63.742 before the first order, and get any averaging scheme approved by the permitting agency and written into the Title V permit before you rely on it (63.743(d)(6)).

Then build the records the semiannual report will need, on day one, rather than reconstructing them six months later.

iSi has run hexavalent chromium and coating compliance programs for aerospace manufacturers for two decades, and the applicability determination is the step where outside help pays for itself most clearly, because it is the step that sets everything downstream. If you are scoping a new line, a new hangar, or a permit modification and want the potential-to-emit calculation and the GG-versus-6H determination done before the equipment is specified, our air permitting team handles that work.

Sources

All regulatory text verified against eCFR on 2026-10-13. Title 40 up to date as of 2026-10-08.

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