Mastic Sealants and Glues

Mastic Sealants and Glues

Without many of the sealants, glues and foams around today the building techniques that we use would be vastly different. But you would be mistaken for thinking that many of these products are a modern invention. For instance, the mastic that we use for gluing or sealing has been around for centuries, and during the ottoman rule of Chios, a Greek island situated in the Aegean Sea, mastic from the mastic tree was worth its weight in gold, and the penalty for stealing it was execution. Of course, the products we now have that involve mastic can be made from several different chemicals that are combined to have a variety of uses, and many of them would be harmful if we were to chew them, which is where the word mastic ( from the Greek word for masticate) comes from.

We are primarily interested in the use of mastics in the construction industry and they are many. Below are just a few of the uses for mastic.

  • Exterior oil based sealant
  • Mastic asphalt is a sticky black highly viscous liquid
  • A putty-like paste used as a joint sealer in building work
  • A construction adhesive used to bond floor, wall and ceiling tiles etc.
  • Stone mastic asphalt used as a surfacing material
  • Decorators caulk for filling cracks and gaps

Types of Mastics Available – In Plastic Tubes for use in a Mastic Gun

Silicone Sealant –

General purpose mastic for forming a waterproof seal between most building materials. – does not stick well to plastic- normal paint will not stick to it- not advisable for filling joints over 8mm wide. Typical use sealing UPVC window frames to brickwork. Ideally only use where the movement of the materials sealed will be minimal. Areas where it is used will tend to smell like your local chip shop for a few hours.

Silicone Sanitary Grade -

As above, designed for forming waterproof seals in bathrooms, has a fungicidal additive to stop mould growth. Typical uses- sealing bath or basin to wall tiling- sealing shower trays to tiles.

High or Low Modulus -

Silicone sealants are often sold with this description on the packaging. 9 out of 10 builders will not know the difference. Well if you’re interested and most people won’t. It’s all to do with the chains of polymer- High is longer chains for more stretch. Low is the opposite. I could also offer you more technical blurb but life is too short.

Glazing Silicone -

Designed to offer more adhesion to non porous substrates.

Roof and Gutter Sealant –

Usually a rubber type sealant that will adhere to Bitumen, Plastic, and most other substrates- used for sealing gutters, cracks in flat roofs, and most have the ability to be used in damp situations. A sod to remove from hands and clothes- wear gloves.

Acrylic Sealants –

Used for sealing wooden frames to brickwork – has the benefit of being able to be painted over. Water based so easy to remove surplus.

Decorators Caulk -

Used to fill cracks in interior situations prior to decorating- Comes in White. Great for sealing gaps in new skirting boards- water based very easy to apply- fill the gap wipe over with a sponge – leave it to dry then paint.

Polyurethane Sealants -

Now we get to the serious sealants. Much harder to use & will stick to just about everything including you. Originally formulated for the Engineering & Automotive industry. Now commonly used for general construction and readily available under many trade names. Far greater tensile strength than any other mastics- Best used in dry conditions will outperform all other mastics available & you can over paint. On the down side -much harder to use especially in cold weather- Tip keep it in a warm room for a few hours prior to use. It’s more expensive to buy- but you get what you pay for. Another Tip when using mastic for filling joints that are likely to expand in width- Use a backing foam ( polyethylene foam is designed for this- comes in all widths and depths – Use it to fill the bottom of the joint leaving it well below the surface – That way the mastic only seals edge to edge.

High Temperature Sealant -

Used to seal fire ducts and any other gap that needs to be protected from heat


Glues have also been around for thousands of years, and most of them used to stink to high heaven when using them. There are several glues that can be found in nature, such as tree resin, or limpets shells stuck to rocks, and bitumen, which could spell death for unsuspecting creatures wandering into a bitumen pool. But today we have turned glues to our advantage.

We use bitumen to construct our roads, and the techniques we use are not too dissimilar from those of the ancient Babylonians. And we still use glues made from the hide, bone or hooves of animals. But the most significant transformation in the formation of modern glues came with the introduction of man-made polymers, but the types of glues available today are as varied as the materials that we use.

Wood glues

Initially made from animal parts, many wood glues are now made from polymers and can be used to assemble joinery products, but most of them have poor gap-filling abilities and also require clamping under pressure to be most effective. There are glues on the market that will expand into joints but may compromise the strength of the joint. Most woodworkers will attempt to use tight-fitting joints that require as little glue as possible to achieve a strong bond.

PVA glue

Perhaps one of the best known glues is PVA (polyvinyl acetate) which is very popular amongst hobbyists and children alike, as it is very easy to use and is also non-toxic. It is not ideal for constant or heavy loads as it does tend to creep or move over a prolonged period of time.

Other Wood Glues to Consider

Powdered Glue -

Based on urea formaldehyde- used extensively in furniture & Joinery manufacture. Mix with water into a paste apply with a brush to both sides of the joint – wipes up easily- Joints require cramping whilst glue sets.

Polyurethane Glue -

A very strong waterproof glue- will also fill gaps. Joints need cramping. On the down side – Messy to use – short shelf life once opened.

Contact Adhesive -

Ideal for bonding laminates & flooring materials, actually will stick pretty much all materials together except plastic and rubber as long as they are dry and tight fitting- Just apply a thin coat to each of the faces with a fine notched applicator. Wait until the adhesive is touch dry and carefully stick together- You won’t get a second chance once its stuck its not coming back off so get it right first time. On the upside no cramping needed.

Glass and ceramic glues

There are several multipurpose glues around that will bond glass and ceramics, but it is better to use a specialized version to achieve the best results. Acrylic latex based cements will have a good resistance to water and heat and are normally sold in tubes for ease of use. You can also use silicone rubber adhesives as they form a very strong bond and will have a good resistance to water and temperature extremes. When using these types of glues it is essential that the surfaces are clean and free from dust.

Metal glues and fillers

Several of the glues used for bonding metal come in two parts. Usually epoxy resin based, when mixed together they create a chemical reaction that hardens the glue, which can often be used as a filler and sealant. Their drying times can vary from 30 minutes to 12 hours, and may take as long as two days to cure or harden completely. Those glues that can also act as fillers can be used to repair gutters, seal pipes or even car repairs.

Plastic glues

Plastic can present a problem when it comes to being glued as many of the solvents found in glues will melt plastic. There are several plastic glues around, some of which are plastic themselves, and so it’s important to know what type of plastic you are dealing with. Model cement will form a strong bond between polystyrenes and acrylics which covers most plastics except for plastic foam. It usually sets in about 10 minutes but can take around 24 hours to cure.

Vinyl adhesive will also form a strong waterproof bond and does not usually require clamping. It sets quite quickly and cures in about 20 minutes.

Acrylic solvent is a bit different as it is not strictly speaking and adhesive and it works by melting the acrylic bonding surfaces together. The surfaces do need to be clamped for the process to work and usually sets in about five minutes.

There are also superglues and contact adhesives which can be used to join plastics. These need a great deal of care when using them as they can easily join fingers together.


Expanding foam products are designed to expand and harden upon contact with the air. They are sold as a DIY product in spray canisters and can be used for a variety of applications. The most popular being gap or hole filling. The fact that they expand can present a large problem to the uninitiated as a whole canister could end up filling a space that is 80 times its original size. It is best to spray a small amount at a time and monitor the expansion so that you don’t overfill the area and have a large amount of foam to cut back once it has hardened.

The canisters also come in waterproof and fire resistant versions, to name just a few. The product is even finding a use in the film and art world where it is used to create lightweight and inexpensive props which are then finished to resemble more solid objects. It is important to wear gloves when using the foam as it will stain your hands black which can take several days to remove.

The insulation industry is also using expanding foam in larger amounts to spray lofts and walls as an effective insulation product.


Image credits: Crown molding, Roadside guitars and Public Domain Photos