MEMORANDUM

DATE: January 6, 2015 

SUBJECT: Technology Review for Chemical Milling Maskant Application Operations in the 

Aerospace Source Category

FROM: Eastern Research Group, Inc. and EC/R Incorporated 

TO: Kim Teal, OAQPS/SPPD/MMG 

1.0 INTRODUCTION 

Section 112 of the Clean Air Act (CAA) requires the U.S. Environmental Protection Agency 

(EPA) to establish technology-based standards for listed source categories. These technologybased standards are often referred to as maximum achievable control technology (MACT) 

standards. The Aerospace MACT (40 CFR part 63, subpart GG) regulates HAP emissions from 

facilities that are major sources and are engaged, either in part or in whole, in the manufacture or 

rework of commercial, civil, or military aerospace vehicles or components. The rule requires the 

use of MACT to reduce HAP emissions from the following operations: cleaning; primer, topcoat 

and chemical milling maskant application; depainting; and waste storage and handling. 

Section 112 also contains provisions requiring the EPA to periodically revisit these standards. 

Specifically, section 112(d)(6) states: 

(6) REVIEW AND REVISION. – The Administrator shall review, and revise as 

necessary (taking into account developments in practices, processes, and control 

technologies), emissions standards promulgated under this section no less often than 

every 8 years. 

To comply with this CAA requirement, the EPA conducted a technology review for the 

Aerospace MACT standard for major sources. This memorandum addresses the portion of the 

technology review for maskant application operations. 

For the purposes of reviewing a MACT standard, the EPA considers a “development” in 

practices, processes, and control technologies to be: 

• Any add-on control technology or other equipment that was not identified and considered 

during MACT development; 

• Any improvements in add-on control technology or other equipment (that was identified 

and considered during MACT development) that could result in significant additional 

emission reduction; 

• Any work practice or operational procedure that was not identified and considered during 

MACT development; and 
• Any process change or pollution prevention alternative that could be broadly applied that 

was not identified and considered during MACT development. 

Section 2 of this memorandum presents a summary of the sources of data that were used to 

conduct the technology review and Section 3 presents the technology review for maskant 

application operations. 

2.0 SOURCES OF AVAILABLE CONTROL TECHNOLOGY INFORMATION

To identify any developments in practices, processes, or control technologies that could be 

applicable to maskant application operations in the Aerospace industry, we consulted the 

following sources of data: RACT/BACT/LAER Clearinghouse, the California Statewide Best 

Available Control Technology (BACT) Clearinghouse, regulatory actions promulgated 

subsequent to the Aerospace MACT, regional and state regulations, and correspondence with 

state agencies and industry. 

In addition, in order to assess developments in practices, processes and control technologies that 

have occurred in the aerospace manufacturing and rework industry since promulgation of the 

MACT standards, the EPA sent out a comprehensive information collection request (ICR) to all 

known facilities in February 2011. The ICR requested information on material usage and control 

techniques used on all of the regulated and unregulated emission sources. Responses were 

received from 100 major source facilities and 145 synthetic minor facilities with adequate 

information to determine the current level of control of HAP emissions, as well as to calculate 

HAP emissions. 

2.1 RACT/BACT/LAER Clearinghouse Database

Under the EPA's New Source Review (NSR) program, if a company is planning to build a new 

plant or modify an existing plant such that criteria air pollution emissions will increase by a 

certain amount, then the company must obtain an NSR permit. The NSR permit is a construction 

permit which generally requires the company to minimize air pollution emissions by changing 

the process to prevent air pollution and/or installing air pollution control equipment. 

The terms "RACT," "BACT," and "LAER" are acronyms for different program requirements 

relevant to the NSR program. RACT, or Reasonably Available Control Technology, is required 

on sources in areas that are not meeting national ambient air quality standards (non-attainment 

areas). BACT, or Best Available Control Technology, is required on new or modified major 

sources in clean areas (attainment areas). LAER, or Lowest Achievable Emission Rate, is 

required on new or modified major sources in non-attainment areas. 

BACT and LAER (and sometimes RACT) are determined on a case-by-case basis, usually by 

State or local permitting agencies. The EPA established the RACT/BACT/LAER Clearinghouse, 

or RBLC, to provide a central data base of air pollution technology information (including past 

BACT and LAER decisions contained in NSR permits) to promote the sharing of information 

among permitting agencies and to aid in future case-by-case determinations. However, data in 

the RBLC are not limited to sources subject to RACT, BACT, and LAER requirements. 
Noteworthy prevention and control technology decisions and information may be included even 

if they are not related to past RACT, BACT, or LAER decisions. 

The RBLC contains over 5,000 air pollution control permit determinations that can help identify 

appropriate technologies to mitigate most air pollutant emission streams. It was designed to help 

permit applicants and reviewers make pollution prevention and control technology decisions for 

stationary air pollution sources, and includes data submitted by several U.S. territories and all 50 

States on over 200 different air pollutants and 1,000 industrial processes. 

 

The RBLC provides several options for searching the permit database on-line to locate 

applicable control technologies. On December 4, 2013, we conducted a search of the RBLC. 

There are no process types in the RBLC for chemical milling maskant application. We then 

specified processes in the Aerospace surface coating category, as surface coating operations also 

include controls similar to those used to control maskant application sources. The search 

included permits dating back to September 1, 1995, which is the promulgation date of the 

Aerospace MACT. The results of this search are presented in Appendix 1. 

2.2 Subsequent Regulatory Actions

No regulatory actions promulgated subsequent to the Aerospace MACT were identified for 

chemical milling maskant application operations. However, various regulatory actions related to 

surface coating operations were reviewed as they include controls similar to those used to control 

maskant application sources. These regulatory actions, listed in Table 1, were reviewed for 

developments in practices, processes and control technologies. 

Table 1. Subsequent Regulatory Actions for Sources Similar to 

Aerospace Coating Operations 

MACT Standards Subpart Date Practices, Processes, and Control Technologies 

Evaluated 

Surface Coating of 

Metal Furniture RRRR 5/23/2003 

Emission limit as mass HAP per volume solids used, 

add-on control devices, low-HAP, high solids 

coatings 

Surface Coating of 

Wood Building 

Products 

QQQQ 5/28/2003 Emission limit as mass HAP per volume solids used, 

add-on control devices, low-HAP coatings 

Surface Coating of 

Large Appliances NNNN 7/23/2002 

Emission limit as mass HAP per volume solids used, 

add-on control devices, low-HAP, high solids 

coatings 

Surface Coating of 

Automobiles and 

Light-Duty Trucks 

IIII 4/26/2004 

Emission limit as mass HAP per volume solids used, 

low-hap coatings (low/ non-HAP solvents), 

improved capture and control systems 

Surface Coating of 

Plastic Parts and 

Products 

PPPP 4/19/2004 

Emission limit as mass HAP per volume solids used, 

add-on control devices, waterborne coatings, lowHAP, high solids coatings
Surface Coating of 

Miscellaneous 

Metal Parts and 

Products 

MMMM 01/02/2004

Emission limit as mass HAP per volume solids used, 

add-on control devices, low-HAP, high solids 

coatings 

Area Source 

Standards Subpart Date Practices, Processes, and Control Technologies Evaluated 

Paint Stripping and 

Miscellaneous 

Surface Coating 

Operations 

HHHHHH 1/9/2008 

Confining all coating operations to a spray booth, 

high transfer efficiency spray guns, training and 

certification of spray equipment operator to optimize 

transfer efficiency 

Residual Risk 

Standards Subpart Date Practices, Processes, and Control Technologies Identified as Developments 

Magnetic Tape EE 04/07/2006 No developments 

Printing and 

Publishing KK 4/21/2011 Permanent total enclosures installed on controlled presses 

Shipbuilding and 

Ship Repair II 11/21/2011

Concentrator/RTO installed on spray booths to 

achieve 95% control efficiency; lower VOC coating 

limits based on California regional rules 

Wood Furniture 

Manufacturing JJ 11/21/2011 RTO; Lower VOC coating limits based on California regional rules, more efficient spray guns 

Developments identified in the regulatory actions listed in Table 1 will be discussed in Section 3 

(Technology Review for Primer and Topcoat Application Operations). 

2.3 State and Regional Regulations

In addition to the RLBC and subsequent regulatory actions, we examined a variety of regulations 

of the Bay Area Air Quality Management District (BAAQMD), the South Coast Air Quality 

Management District (SCAQMD), the San Diego County Air Pollution Control District 

(SDAPCD), the Ventura County Air Pollution Control District (VCAPCD), and the Missouri 

Department of Natural Resources. These regulations generally address VOC emissions, rather 

than volatile organic HAP emissions, but in many cases include provisions to reduce organic 

evaporative emissions from maskant application just as intended for the chemical milling 

maskant affected source under the Aerospace MACT. 

The SCAQMD and the SDAPCD provide VOC content limits for Type I and Type II chemical 

milling maskants, similar to the format of the Aerospace MACT. Both California rules specify a 

content limit for Type I chemical milling maskants of 250 grams of VOC per liter (g/L) of 

coating applied, which is more stringent than the VOC limit of 622 g/L for Type I maskants set 

forth in the Aerospace MACT. Note that the emission limits for VOC and HAP are identical in 

the Aerospace MACT standards for maskants (i.e., the HAP limit for Type 1 maskants is also 

622 g/L). Because volatile organic HAP is a fraction of total VOC, an emission limit for HAP 

that is equivalent to the VOC limit of 250 g/L VOC in the SCAQMD and SDAPCD rules would 

be equal to or lower than 250 g/L. 
All four California district rules reviewed have more stringent requirements for sources choosing 

to control emissions through the use of an add-on control device. These rules specify at least an 

85 percent combined capture and control efficiency, compared to the 81 percent capture and 

control requirement in the Aerospace MACT. 

The SDAPCD aerospace coatings standard provides additional requirements for maskants 

applied by dip coating application equipment that are not specified in the Aerospace MACT. 

This standard includes the following requirements: 

(6) Maskant Dip Coating Application Equipment. 

Except as provided in Subsections (b)(1), (b)(6), and (b)(7), a person shall not use a dip 

tank to apply Type I chemical milling maskants or maskants for chemical processing or 

component coatings of a multi-stage maskants to aerospace parts unless: 

(i) The dip tank is covered except when being accessed to add or remove 

materials; take samples; visually inspect the maskant level; clean, maintain or 

repair the tank; or apply maskant; and 

(ii) The dip tank has a readily visible, permanent mark or line indicating the 

maximum allowable maskant level; and 

(iii) The dip tank has a freeboard ratio greater than or equal to 0.5; and 

(iv) Maskant agitation is achieved by means other than gas agitation; and 

(v) Material is added to the dip tank by means of submerged filling; and 

(vi) Any dip tank lip exhaust ventilation system with an inlet located below the 

cover of the maskant application dip tank is turned off and the ventilation duct 

closed when the maskant application dip tank is covered. 

We did find that some California air quality management districts have more stringent VOC 

content limits than those in the Aerospace NESHAP. However, the EPA is not aware of any 

facilities located in these two districts using chemical milling maskants and that would be subject 

to these more stringent limits. 

3.0 TECHNOLOGY REVIEW FOR CHEMICAL MILLING MASKANT 

APPLICATION OPERATIONS 

In the process of chemical milling, an etchant solution is used to reduce the thickness of selected 

areas of metal parts. The process is typically used when the size or shape of the part precludes 

mechanical milling or when chemical milling is advantageous due to shorter processing time or 

its batch capability. Before chemical milling, a maskant is applied to the part, allowed to cure, 

and is then removed from selected areas of the part where metal is to be removed by the etchant. 

The maskant remaining on the part protects those areas from the etchant. Maskants are applied 
by brushing, dipping, spraying, or flow coating. Organic HAP emissions occur through 

evaporation of the solvent, typically toluene, xylene or perchloroethylene, as the maskant is 

applied and while it cures. 

3.1 Summary of Existing MACT Level of Control 

The Aerospace MACT standard (subpart GG) promulgated in 1995 requires the following 

organic HAP and VOC content limits for uncontrolled chemical milling maskants: 

• Type I: 5.2 pounds organic HAP per gallon (622 g/L) less water, as applied, 

• Type II: 1.3 pounds of organic HAP per gallon (160 g/L) less water, as applied.

These requirements do not apply to touch-up of scratched surfaces or damaged maskant and 

touch-up of trimmed edges. Alternatively, a control system can be used to capture and control 

emissions from the maskant application operation. The system must achieve an overall control 

efficiency of 81 percent. 

3.2 Identified Control Measures for Chemical Milling Maskant Application Operations 

Current industry practices employed to control emissions from chemical milling maskant 

application operations include the use of a control device, such as oxidizers and carbon 

adsorbers, and product substitutions, such as waterborne maskants. 

3.2.1 Add-On Control Technology or Other Equipment Not Identified and Considered During 

MACT Development

No new equipment or control technologies were identified that were not considered during 

MACT development. 

3.2.2 Improvements in Add-On Control Technology or Other Equipment (That Was Identified 

and Considered During MACT Development)

The MACT floor HAP content limit of 1.3 lb/gal for Type II maskants was based on the 

emission rate of the best performing 12% of sources, which was represented by two sources. The 

emission rate accounted for the control measures used at each facility. One of these sources 

utilized a carbon adsorber and the other source utilized a waterborne chemical milling maskant. 

Each source had the same organic HAP emission rate of 1.3 lb/gal. Advancements in the use of 

add-on control technologies to achieve significantly higher emission reductions than required by 

the current MACT limits could constitute a development as an improvement in add-on control 

technology. 

A summary of the controls reported in the 2011 ICR for chemical milling maskant application 

operations and the average percent reduction from the controls, is provided in Table 2. 

 
Table 2. Control Measures Reported in the 2011 ICR for Chemical Milling Maskant 

Application Operations 

Control Measure Percent Reduction 

Carbon adsorber, catalytic oxidizer 98 

Zeolite concentrator, carbon adsorber, catalytic oxidizer 95 

Carbon Adsorber 92 

The types of controls that are reflected in the data collected through the 2011 ICR are the same 

types of controls that were considered in the development of MACT and which are presented by 

the requirements in 40 CFR 63.747(d) to achieve 81 percent or greater combined capture and 

destruction or removal efficiency. South Coast Air Quality Management District (SCAQMD) 

rule 1124 includes requirements for the use of add-on control technology of at least 90 percent 

capture and 95 percent destruction or removal, for an overall control efficiency of 85.5 percent, 

compared to the overall control of 81 percent in the Aerospace MACT. The requirements in 

SCAQMD rule 1124 and the Aerospace MACT are based on the same control technology, a 

carbon adsorber; however, the Aerospace MACT does not specify the type of control technology 

required. The difference in the overall minimum control efficiency provided by SCAQMD and 

the EPA is a result of differences in assumed capture and destruction efficiencies and not in the 

actual performance of the control equipment. Therefore, no improvements in add-on control 

technology or other equipment were identified. 

3.2.3 Work Practices and Procedures Not Identified and Considered During MACT 

Development

No additional work practices or procedures were identified that were not already identified and 

considered during MACT development. 

3.2.4 Any process change or pollution prevention alternative that could be broadly applied that 

was not identified and considered during MACT development.

Pollution prevention measures that were considered during MACT development include product 

reformulations for coatings, including waterborne and lower HAP maskants. No further 

advancements in the development or adoption of waterborne or lower HAP maskants have been 

identified. The “uncontrolled” HAP content of the maskants reported in the 2011 ICR are 

consistent with those in use during the MACT development. Rather than using low-HAP 

coatings, facilities are relying on add-on control technologies to reduce emissions.