How to Apply Sealant

Updated April 17, 2017

Mastic sealant is a great way of filling gaps between two materials in order to create a flexible and waterproof bond. It is often used in bathroom areas, where changes in air humidity and temperature can cause materials to expand and shrink. Mastic allows this to happen without causing cracks or gaps, which can be unsightly. Applying mastic sealant is a very straightforward process, but does require practice.

Purchase an applicator gun from your local hardware store. This allows you to create a neat and tidy joint in an easy-to-use way.

Decide which sealant you need, based on the work you need to carry out. From general-purpose sealants, to sanitary sealants, to sealants that are used for your roof, each sealant type has distinct qualities. Make sure you define beforehand which sealant you need.

Using a sharp craft knife, put the nozzle provided with the mastic tube on a hard flat surface. Cut the nozzle at a 45-degree angle. The closer to the end of the nozzle you cut, the thinner the mastic bead that you will apply. Make sure you start small and work your way up to the thickness you require.

Wipe the surface that you will be sealing with a damp cloth to remove any dust and debris.

Screw the nozzle onto the end of the tube of sealant and insert it into the applicator gun. Gently squeeze the trigger until you see the mastic nearing the end of the nozzle. Place the nozzle in the area you want to seal and gently pull the trigger, moving the applicator to ensure the bead is the same thickness throughout its length.

Dip your finger in water and press firmly over the length of mastic to push and shape it into the joint. You can use a specialist sealant smoother to carry out this function, if you have one.


Buy an extra tube of sealant in order to practice beforehand. Sealant is very difficult to remove when wet, and can cause a lot of mess. Cover up all nearby furniture to protect it.


Moving the gun too fast will stretch the bead of mastic; moving it too slowly will cause it to bunch and distort. Make sure you move at a slow, uniform speed.

Things You'll Need

  • Applicator gun
  • Mastic tubes
  • Craft knife
  • Damp cloth
  • Sealant smoother
Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

Ben Wakeling graduated from Coventry University in 2009 with an upper second class honours B.Sc. degree in construction management. Wakeling is also a freelance writer, and works for a number of businesses, such as Demand Studios, Suite 101 and Academic Knowledge.