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Building facades endure constant movement and weather exposure. Getting the weather seal right prevents future call-backs and costly water ingress. DOWSIL 791 delivers robust, premium performance weather sealing for critical expansion, connection, and perimeter joints. This neutral-cure silicone sealant offers significant movement capability and long-term durability, ensuring the integrity of curtain walls and building envelopes.
DOWSIL 791 is formulated as a workhorse weather sealant for high-performance building envelopes. It's engineered to accommodate the dynamic forces exerted on facades, including thermal expansion, wind load deflections, and building settlement. This neutral-cure RTV silicone forms a flexible, durable rubber seal that stands up to harsh environmental conditions, providing peace of mind for architects and contractors needing reliable, long-lasting weather protection.
DOWSIL 791 extrudes easily and offers a 20-minute working time, allowing for precise application and tooling across complex joint geometries. Its tack-free time of 45 minutes provides sufficient window for final adjustments before skinning. For optimal performance and to prevent three-sided adhesion, always install a compatible backer rod to control sealant depth. While primer is often not required on non-porous surfaces, a test placement is recommended, and an appropriate primer should be used on certain porous substrates for optimal adhesion. Tool the sealant within 15 minutes of application, then remove masking tape immediately for clean lines.
DOWSIL 791 is purpose-built for the rigorous demands of exterior building applications where movement and weather exposure are constants.
While both DOWSIL 791 and DOWSIL 795 are high-performance silicone sealants for building applications, their primary use cases differ significantly. DOWSIL 791 is specifically engineered as a premium weatherproofing sealant for facade joints, offering exceptional movement and weather resistance. It is the go-to for sealing expansion, connection, and perimeter joints where preventing water ingress is paramount. In contrast, Dowsil 795 Silicone Sealant is a dedicated structural glazing sealant, designed for applications where the sealant bears a structural load, such as bonding glass or panels to a frame without mechanical fasteners. Do not interchange these products. Select DOWSIL 791 for robust weather seals, and DOWSIL 795 when structural performance is required.
Dowsil 791 provides ±50% joint movement capability, making it ideal for expansion joints, perimeter seals, and other high-movement façade elements. This flexibility helps accommodate structural shifts due to thermal expansion, wind loading, or seismic activity without compromising seal integrity.
Yes, Dowsil 791 is compatible with many porous substrates such as masonry and concrete. While primers are generally not required, Dow recommends test placements on porous surfaces for critical applications to ensure adhesion and staining performance.
The working time for Dowsil 791 is approximately 20 minutes, and the sealant becomes tack-free in around 45 minutes under standard conditions (23°C, 50% RH). Always tool before skin formation to ensure optimal adhesion and joint aesthetics.
No. Dowsil 791 is not recommended for below-grade or permanently immersed applications. It requires atmospheric moisture to cure and may not perform reliably under continuous water exposure. Consider using an alternative structural sealant for submerged joints.
No. Silicone sealants like Dowsil 791 are not paintable. Paint will not adhere to cured silicone. If a paintable joint is required, consider using a hybrid or polyurethane-based sealant designed for coating compatibility.
Yes. Dowsil 791 meets major international standards including ASTM C920 Class 50, ISO11600 F&G-25LM, EN15651-1/2:2012, and GB/T14683-2017. These certifications confirm its durability, adhesion, and movement capability across various climates and construction types.
| Application / Use Case | Key Benefit | Primary Trades |
|---|---|---|
| Perimeter sealing of windows and doors | Accommodates large thermal movement with ±50% flexibility | Glaziers, Builders |
| Curtain wall and façade joints | UV- and weather-resistant for long-term external durability | Facade Installers, Architects |
| Precast concrete panel joints | Excellent adhesion to masonry and porous substrates | Commercial Builders, Remedial Contractors |
| Metal panel expansion joints | Neutral cure won’t corrode galvanised or coated metals | Sheet Metal Workers, Roofing Contractors |
| Weatherproof sealing of complex building geometries | Extended tooling time allows detailed joint work | On-site Applicators, Custom Builders |
Ensure all substrates are clean, dry, and free from contaminants such as oil, dust, release agents, and old sealants.
Wipe surfaces with isopropyl alcohol or a recommended non-oily solvent using a lint-free cloth. Replace cloths frequently to avoid redepositing contaminants.
Use wire brushing or light abrasion to remove laitance or loose particles from masonry or oxidised metals. Blow out joints with compressed air.
Ensure joint design allows for movement, with a width-to-depth ratio of 2:1. Install a backer rod to control depth and prevent three-sided adhesion.
Use non-absorptive, closed-cell rod to prevent bond to the back surface of the joint and allow movement accommodation.
In shallow joints where rod isn't possible, apply bond-breaker tape to the base to prevent three-sided bonding.
Apply Dowsil 791 using a caulking gun in a steady, continuous bead. Ensure full contact with both sides of the joint and backer rod.
Use a standard or pneumatic caulking gun compatible with 300ml cartridges. Apply even pressure and avoid gaps or air bubbles.
Use a barrel gun to apply 500ml sausages for high-volume jobs. Adjust nozzle to suit joint width and angle for best results.
Tool the sealant to ensure it wets the substrate and achieves full contact. Remove masking tape immediately after tooling. Allow to cure naturally with ambient moisture.
Use a rounded spatula or jointing tool to shape the bead. Do not add water or solvents during tooling.
Typical full cure takes 7–14 days at 25°C and 50% RH. Avoid water exposure during first 24 hours.
Wear chemically resistant gloves (e.g., nitrile), safety glasses with side shields, and long sleeves to minimise skin contact. Avoid handling without protection.
Ensure adequate ventilation during application and curing, especially in enclosed or poorly ventilated areas. Avoid breathing vapours directly.
Although not classified as flammable, avoid exposing sealant to ignition sources during application. Keep containers sealed when not in use and store below 30°C.
Seek medical attention immediately in all cases of serious exposure.
Clean glass thoroughly using a non-oily solvent and lint-free cloth. Ensure surface is dry and free from dust, grease, and protective coatings.
Adhesion testing recommended for coated or reflective glass types.
Wipe with isopropyl alcohol or an appropriate cleaner to remove oily residues and dust. Use clean cloths to prevent cross-contamination.
Ensure surfaces are fully cured and not chalking. Test adhesion on unknown coatings.
Ensure surface is dry and cured for minimum 28 days. Remove laitance, dust, or loose particles using wire brushing or compressed air.
Dow primer may improve adhesion where surface conditions vary. Always test for staining on natural stone.
Dow 1200 OS PrimerWipe with solvent and dry cloth to remove zinc salts or oils. Slight abrasion may be used to expose fresh metal if corroded.
Ensure compatibility with surface treatments or coatings.
Clean using a pH-neutral cleaner and allow to fully dry. Avoid use on freshly honed or unsealed stone.
Sealant should pass ASTM C1248 stain testing before full application. Use only approved primers.
Dow Stone Primer (as tested)| Property | Value |
|---|---|
| Base Chemistry | One-part, Neutral-cure RTV silicone |
| Curing System | 7–14 days @ 25°C, 50% RH |
| Colour | Black |
| Density | 1.51 g/cm³ |
| Consistency | Non-sag (0 mm slump) |
| Tensile Strength | 3.999 MPa (0.40 MPa after water immersion) |
| Elongation | 430% |
| Movement Capability | ±50% |
| Shore Hardness | 25–30 |
| Pot Life | 20 minutes |
| Initial Cure | 45 minutes @ 23°C, 50% RH |
| Application Temperature | 4°C to 40°C |
| Service Temperature | -50°C to 150°C |
| Coverage Rate | 210 g/minute (flow rate) |
| VOC Content | 45 g/L |
| Certifications | ASTM C920 Type S, Grade NS, Class 50, Use NT, M, G, A; ISO 6427°C&G-25 LM; EN15651-1/2:2012; GB/T14683-2017; KS F4910°C to 1099°C-25 LM |
| Standards | 9.0 kN/m (adhesion) |
| Packaging | 500 ml sausage, Carton of 20 |
| Shelf Life | 12 months (stored below 30°C) |
Here are some common examples on where Dowsil 791 Weatherproofing Sealant used by our customers.
