Foster® Fire Resistive Adhesive 81-33

1 of 184 products in this brand
Foster® Fire Resistive Adhesive 81-33 is a high-quality adhesive designed for securely bonding thermal insulation to various structural surfaces. This quick-setting adhesive exhibits excellent specific adhesion to materials such as iron, steel, aluminum, and most types of thermal insulation. Notably, it can be employed without the need for mechanical fastening, making it suitable for marine hull insulation applications.

Product Type: Adhesive

Application Area: Joints

Compatible Substrates & Surfaces: Aluminum, Iron, Metal, Polyurethane (PU) Foam, Stainless Steel

Application Method: Trowel

Features: Fast Setting, Fire-Resistant, Humidity Resistant, Water Resistant

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Identification & Functionality

Product Type

Features & Benefits

Product Highlights
  • Fire Resistive Adhesive is the preferred adhesive for use with rigid polyurethane insulations.
  • It can be used as a fabricating adhesive in making fittings with polyurethane insulation, and as an adhesive to attach insulation to equipment surfaces.
  • Bonds of polyurethane to itself made with 81-33 are stronger than the base insulation.
  • Fire Resistive Adhesive withstands continuous surface temperatures up to 300°F (149°C), but it has been subjected to higher temperatures for short intervals with good results.
  • Its excellent resistance to water and high humidity makes it a good sealing compound for joints and seams of insulation on cold water piping and equipment.
  • Fire Resistive Adhesive meets the requirements of Military Specification MIL-A-3316C, Class 3, Grade A.
  • Fire Resistive Adhesive contains no asbestos, lead, mercury, or mercury compounds.

Applications & Uses

Application Area
Application Method
Preparation and Application Information

Material Preparation

  • Remove paper disk and any skin on surface of adhesive in container.
  • Stir well.
  • Do Not Thin.
  • Apply only to clean, dry surfaces.
  • Keep container closed when not in use to prevent solvent evaporation.


  • Galvanized metal must first be treated with Foster Primer 40-26 and allowed to dry.
  • Attachment Adhesive - Apply with notched trowel at 40 sq. ft./gal. (1.0 m2 /l).
  • Joint Sealant / Adhesive - Apply with smooth trowel at 25 sq. ft./gal. (0.6 m2 /l). Installllllll insulation applying pressure to assure complete and uniform contact to the surface, twisting the insulation section slightly to break any skin that may have formed on the adhesive surface.
  • Square Notched Trowel Use a steel trowel with square teeth 1/8 inch wide, 1/8 inch deep and 1/8 inch apart. Trowel with firm pressure leaving ridges of adhesive. Press insulation firmly into place to obtain complete contact. On rough or porous surfaces a thicker application of adhesive will be required in order to assure contact between the insulation and the substrate.

Clean Up Use a solvent such as chlorinated solvent (non-flammable) or xylene (flammable).

Coverage Range

Wet coverages shown below are for smooth non-porous surfaces. Porous or rough surfaces may require high gallonage to attain required dry thickness. 1.33 to 1.25 gal./100 sq. ft per coat. 0.021 to 0.020 in. wet film thickness (Subject to nature of material being coated.)


Physical Form
Typical Properties
ValueUnitsTest Method / Conditions
Weight/U.S. Gallon13.2lbs.ASTM D1475
Coverage Range25 - 40sq.ft./gal.-
Bonding Time Range2 - 10MinutesFSTM 66
Wet Flammability43.0°CASTM D3278
Application Properties
ValueUnitsTest Method / Conditions
Application Non-volatile (by Volume)66.0% ASTM D1644
Application Non-volatile (by Weight)80.0% ASTM D1644
Service Temperature Range-59 to 149°C-
Flame Characteristics
ValueUnitsTest Method / Conditions
Flame Spread ¹10ASTM E84
Smoke Developed ¹5ASTM E84

¹ - Tested at coverage rate of 40 sq. ft./gal. Applied to 1/4 inch (6.4 mm) inorganic reinforced cement board. The flame spread may vary at different product thicknesses and/or when applied over other surfaces.

Regulatory & Compliance

Storage & Handling

Shelf Life
6 months
Handling Conditions
  • Store and apply between 40°F (4°C) and 100°F (38°C).
  • Always test plastic materials for compatibility when using a solvent base product.
  • Make certain this product is completely dry, and the area is free from solvent odor if food is involved.
  • Galvanized metal must be primed with Foster 40-26.
  • If used between impermeable surfaces, drying time will be extended.