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Non-drying mastic used as a waterproof seal for gutters, windows and doors.

Product overview

Bostik Powermastik is a non-drying, oil based mastic, which provides a durable, waterproof seal for gutters, roof bolts, ducting, windows and doors. It forms a protective self-healing skin on the surface keeping the body of the mastic permanently flexible in order to accommodate any structural or thermal movement on the surface. If the skin is cut, the oils beneath will flow and reseal the damage. It remains flexible over a wide range of temperatures, and has excellent adhesion to most materials used in construction. Exhibits excellent adhesion to PVC, polycarbonate, fiberglass, aluminium, steel, galvanized iron, tin plate and other non-ferrous metals, brick, natural stone, glass, painted and varnished surfaces, ceramic tiles, sand cement screeds, concrete, plaster and timber.

  • Sealing of PVC, galvanized iron and aluminum gutter sections & down pipes.
  • Sealing overlap joints of roofing sections, air conditioning ducting joints and seams.
  • Sealing of windows and door frames.
  • Sealing of roof bolts.
  • Placing window sills, frames and other fittings where a waterproof seal is needed.

Do not use for floor joints or on bituminous surfaces. Not recommended for high movement joints. Not suitable for continuous immersion in water. Not suitable as an adhesive to bond two surfaces together. Do not use where sealant strength is needed e.g. to hold guttering in position. Application onto porous surface may reduce longevity of the product due to absorption of the oils into the surface.

Can be painted to achieve the desired colour.



  • Bostik DIY South Africa Badge Waterproof
  • Bostik DIY South Africa Badge Solvent free
    Solvent free
  • Bostik DIY South Africa Badge Non toxic
  • Bostik DIY South Africa Badge Low shrinkage
    Low shrinkage
  • Bostik DIY South Africa Badge Paintable
  • Bostik DIY South Africa Badge Permanently flexible
    Permanently flexible
  • Bostik DIY South Africa Badge Interior Exterior
    Interior & Exterior
  • Bostik DIY South Africa Badge UV Resistant
    UV Resistant

Documents or Resources


  • Ensure that surfaces are clean, dry and free of loose materials, dust, grease, rust and other contaminants.
  • Soaps or detergents used to clean the surface must be rinsed away thoroughly with clean water to ensure that all traces of the soaps are removed before sealing.
  • Use masking tape to get a clean, even sealant line and to eliminate cleaning difficulties on porous surfaces.
  • Remove tape after mastic application.

280ml Cartridge

  • Cut the tip off the cartridge and screw on the nozzle.
  • Cut the tip of the nozzle at an angle to achieve the desired bead size.
  • Apply with a caulking gun.

410ml Cellopak

  • Apply mastic with either a closed barrel caulking gun, putty knife or spatula.
  • Apply the mastic to the surface, and smooth down using a flat or rounded tool.

Clean tools and excess mastic with lacquer thinners or turpentine.

Skin formation takes approximately 4 to 7 days, depending on environmental conditions. High humidity and low temperatures will extend skinning time.

It is recommended that for exposed applications, the mastic be painted with a non-bituminous paint after skin has formed, to prolong the service life.

Frequently Asked Questions

Wet and dried adhesive on applicator tools can be cleaned with acetone, thinners or parrafin. Please refer to the Technical Data Sheet of the specific product for more information.

To maximize the shelf life of an opened silicone cartridge, we recommend that the nozzle be removed and a piece of plastic placed over the cartridge tip after which the nozzle must be screwed back on. A large screw inserted into the nozzle tip also helps to extend the life. Store in a cool environment.

Stuck in a rut and not sure which sealant is suitable for your application? Call our Sticky Questions Helpline during office hours on 0800 222 400 for technical support.