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Written By: Daniel King, Last modified: February 18, 2021
Quick Facts
  • Places Used:
    Ships, homes, offices, boilers, furnaces, steel mills, factories
  • Toxicity:
  • Friable:

Mastic is paste-like cement that is used as an adhesive, sealant and joint-filler. Gunning mix is a type of paste applied to metals, plastics, wood or masonry as a repair compound. It may also be used to smooth pitting, rough surfaces or irregularities that may occur in equipment lining. This mix is usually applied using a caulk gun or similar pump gun.


Adhesives like mastic and gunning mix have been used for hundreds of years, but the use of modern construction mastics and gunning mix began in the early 1900s. Many new plastics and rubbers were synthetically produced and several new adhesive products were developed for use in the construction industry.

Many of these mixes were produced for use during World War II. Adding synthetic rubber and other fibers to these mixes allowed them to be more flexible and strong. Asbestos was added to these adhesives to make them resistant to extreme temperatures and chemicals.

Construction mastic may be used in residential homes or commercial buildings in areas such as:

  • Under floor tiles
  • Ceiling tiles
  • Acoustical tiles
  • Roofing
  • Walls
  • Windows

Gunning mix is commonly used to fill in gaps or spaces in the surfaces of the following equipment:

  • Steel furnace lining
  • Hulls
  • Storage tanks
  • Sonar domes
  • Pump casings

After the use of asbestos declined because of health concerns in the 1980s, companies began using alternatives to asbestos in their construction mastic and gunning mix products.

Companies that manufactured construction mastics and gunning mix include:

  • Amchem, Inc. (Benjamin Foster Company)
  • Harbison-Walker Refractories Company
  • Insul-Mastic Corporation of America
  • North American Refractory Company
  • 3M (Minnesota Mining and Manufacturing Company)
  • Owens-Corning Fiberglass Corporation
  • Johns Manville


The National Emission Standards for Hazardous Air Pollutants considers construction mastic as a Category I non-friable asbestos-containing material. This classification indicates that mastic contains 1 percent or more asbestos and may emit asbestos fibers if disturbed.

When working with asbestos-containing mastic and gunning mix, the following activities may cause it to release fibers:

  • Grinding
  • Sawing
  • Drilling
  • Cutting
  • Abrading
  • Renovating
  • Demolition

Gunning mix was often made with vermiculite and aluminum. The vermiculite used in gunning mix may be contaminated with asbestos. Before the mix is made into a paste, it comes in powder form and is packaged in bags. When bags are opened, asbestos fibers easily become airborne while gunning mix is measured and mixed.

Construction workers who install, remove, repair or perform other maintenance work may have been exposed to asbestos in mastics and gunning mixes. Workers in the factories that produced these products were also exposed asbestos on a daily basis.

Occupations that are at risk of exposure from mastics and gunning mixes include:

  • Steel mill workers
  • Power plant employees
  • Engineers
  • U.S.veterans
  • Shipbuilders
  • Maintenance workers
  • Carpenters
  • Flooring installers

As a result of asbestos exposure from mastics and gunning mixes, a number of lawsuits have been filed against several manufacturers of these products.


In the 1990s, two engineers filed lawsuits against the maker of NARCO Lite Gunning Mix, North American Refractory Company (NARCO). Frederick Moss and Martin Easter were exposed to high levels of asbestos dust when products were mixed. Witnesses testified that a number of NARCO products, including the gunning mix, released a high level of asbestos dust. No workers were wearing respiratory protection.

As a result of the asbestos exposure, both Moss and Easter developed mesothelioma. Moss worked as an engineer from 1946 to 1988 at Alabama Power Company. Easter was an engineer with U.S. Steel from 1941 to 1970. The jury found North American Refractory Company liable and awarded $7 million dollars to Moss, Easter and a third plaintiff who suffered from asbestosis.

The amount of lawsuits forced NARCO into bankruptcy and when it emerged from bankruptcy in 2008, the North American Refractories Company Asbestos Personal Injury Settlement Trust was created to pay future asbestos claims. The trust was funded with $6.32 billion.

Abating the Product

Asbestos-containing construction mastic and gunning mix may be applied to a variety of household products such as floor tiles or roofing materials. Some states allow homeowners to abate mastic and gunning mix that is whole and intact, but this is not advised unless the homeowner is fully prepared to take all the legal and safety precautions. Mastic and gunning mix that has been applied to insulation board or lagging should be removed by a certified asbestos abatement contractor.

Keep in mind that many states require homeowners to use professional, trained contractors when removing asbestos products. The EPA also recommends that only licensed, trained professionals abate asbestos materials. Asbestos abatement professionals also prevent homeowners from paying hefty fines if they break federal or state asbestos abatement regulations.

Homeowners can use the following guidelines to ensure professionals are adhering to laws and regulations when removing asbestos-containing mastics and gunning mixes.

Hired asbestos abatement professionals should:

  • Remove all furniture or personal items from the room
  • Cover nearby objects with sheeting and seal off the space to avoid contamination
  • Wear proper safety equipment (HEPA respirator, coveralls, gloves, shoe protectors and eye protection)
  • Scrape off mastic that has been softened through heating or applying a solvent material
  • Avoid creating dust or scattering pieces of mastic (wetting the material reduces dust)
  • Wipe down surfaces and clean the area with a HEPA vacuum
  • Seal debris in polythene bag and dispose of the materials in an asbestos-approved waste location


Some brands of these products include:

  • Castable Mix 204
  • H-W Lightweight Castable #10
  • NARCO Aerogun
  • NARCO Lite Castable
  • NARCO Lite Gunning Mix
  • Owens-Corning Type II Mastic
  • Steelplant Castable B

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