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For general glazing and weather sealing in curtain wall installations and building facades, Dowsil 791 Weatherproofing Sealant provides a durable, flexible barrier that accommodates high-movement perimeter and expansion joints. This one-part, non-sag silicone cures at ambient temperature by reacting with atmospheric moisture, forming a resilient rubber seal without requiring a primer on most non-porous surfaces.
Dowsil 791 is a neutral-cure RTV silicone formulation. Because it does not release corrosive byproducts during cure, it is suitable for direct contact with sensitive building components including coated glass, galvanised steel, and masonry. Once cured, the silicone structure is virtually unaffected by sunlight, rain, snow, or ozone, maintaining its bond and elasticity under direct UV exposure and thermal weathering. The raw sealant has low stringing upon gunning and zero slump, allowing clean application in vertical and overhead joints.
This sealant is formulated for weatherproofing joints across key structural facade substrates. It forms an unprimed bond to most non-porous surfaces, while certain porous substrates may require a primer for optimal adhesion. Suitable substrates include:
Dowsil 791 must be used within its performance boundaries to prevent joint failure:
Before applying Dowsil 791, clean all joint surfaces of foreign matter, grease, oil, dust, water, frost, surface dirt, old sealants, or protective coatings. Install backing material, joint filler, setting blocks, or spacer shims as specified. Mask adjacent areas to ensure neat tooling lines. Apply the sealant in a continuous operation using positive pressure. Tool the sealant with light pressure within 15 minutes of application—before a skin forms—to spread the material firmly against the backing rod and joint faces. Remove masking tape immediately after tooling.
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.
