H.B. Fuller Extra/Build® Mirror Mastic

H.B. Fuller Extra/Build® Mirror Mastic is a safe, permanent bond for professional mirror installations. Extra/Build® allows you to build up and fill gaps up to 2½" without any sag. Extra/Build® is blended to achieve maximum build with strong adhesion between mirror and drywall, wood, metals, and many more surfaces.

Product Type: Contact Adhesive, Mirror Mastic

Application Area: Mirror, Wall

Compatible Substrates & Surfaces: Concrete, Metal, Wood

Technical Data Sheet
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Identification & Functionality

Features & Benefits

Key Attributes
  • Easy Working Consistency Year-round - At temperatures ranging from 10°F to 100°F, Gunther Extra/Build® is easily gunned from cartridges/sausages or applied from one gallon cans or five gallon pails.
  • Adjustable - Extra/Build® allows mirrors to be adjusted for perfect alignment and it keeps them aligned.
  • Strong Permanent Bond - Extra/Build® is blended to achieve maxium build with strong adhesion between mirror and drywall, wood, metals, and many more surfaces.
  • Quality Proven - Extensive testing proved Gunther Extra/Build® holds and lasts.
  • Convenient Drying -  At normal room temperature Extra/Build® cures to about 20% of maximum strength in 24 hours.

Applications & Uses

Application Area
Compatible Substrates & Surfaces
Application Instructions

Installation Instructions

  1. The surface must be clean, dry and free from dust, dirt, greasy film, loose or powdery paint or primer. For assurance sand to the base surface. Clean and seal with Gunther Prime-N-Seal™, let fully dry. When applying mirror to vertical unpainted plaster, wallboard, wood, particle board, hardboard and concrete, the surface to be covered must be sealed with Gunther Prime-N-Seal™. Do not apply on wallpaper, painted surfaces, vinyl safety backing, acrylic mirror, or treated woods. Not for external applications. A sealer will add strength to the wall and prevent caustic lime deposits from attacking the mirror backing. Wall and mirror surfaces must be above 60°F to prevent condensation of moisture. For additional information and Installation procedure, refer to Specification Data Sheet.
  2. For Best Application Results: Apply Gunther Extra/Build® in 4” diameter patties for every square foot. Not for exterior applications. Do not use on acrylic mirrors.
  3. Use a mechanical support system to support mirror during and after installation.
  4. Gap Filling Capabilities: Extra/Build® can obtain gap fill of 2” to 2½” without sagging at a wide range of temperatures.
  5. Application: Extra/Build® adhesive is easily applied at temperatures ranging from 10°F to 100°F without auxillary heating equipment. Extra/Build® can be easily gunned from cartridges or applied from one gallon cans.
  6. Extra/Build® is the only mastic that can be applied in 4-5” diameter patties.

H.B. Fuller Extra/Build® Mirror Mastic - Application Instructions

Do not apply mastics in a circle where an air pocket could collect solvents and possibly extract material that could be damaging to the mirror. The best way to apply Extra/Build® from a cartridge is to start from the center and let the mastic naturally expand out as it’s being gunned. Mechanical support systems are recommended for all mirror applications. This includes the use of mechanical fasteners at the top and bottom of the mirror.

Skin Rate

Gunther Extra/Build® will develop a skin within 30 seconds and rewet upon application. When the mirror is set in place on the wall, the surface skin will dissolve for maximum contact with both the mirror backing and wall. Once mirror is placed on the wall DO NOT REMOVE. “Air Tacking” is not necessary.

Adjusting Time

Extra/Build® can be adjusted for a period of 24 hours after installation to ensure proper mirror installation.

Mounting Time

Mirrors can be mounted up to 20 minutes after application of Extra/ Build®.


For easy cleanup, mineral spirits will remove uncured Gunther Extra/Build®. Test the material to be cleaned to make sure mineral spirits will not cause any damage.

Cure Rate

Rate Of Curing For Gunther Extra/build®

Time Cured

Approximate Percent of

Maximum Strength

24 hours 20%
3 days 50%
7 days 100%
30 days 100%

Safety & Health


FLAMMABLE vapors from Extra/Build® may ignite explosively; prevent build-up of vapors. Open all windows and doors. Use only with cross ventilation. Do not smoke. Extinguish all flames and pilot lights and turn off stoves, heaters, electric motors and other sources of ignition during use and until all vapors are gone. Close container after use. Keep away from heat, sparks and open flame.

Packaging & Availability

  • 20 fl oz sausage
  • 300 ml cartridge
  • 1 gallon can
  • 5 gallon pail

Storage & Handling

Shelf Life
1 Year
Storage Conditions

Shelf life is one year in original unopened containers when stored between 40°F - 80°F.