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Dowsil 791 Silicone Weatherproofing Sealant: A Technical Overview
Dowsil 791 is a premium, one-part, neutral-cure RTV silicone sealant engineered for long-term weatherproofing in building envelope systems. With ±50% joint movement and excellent resistance to UV, ozone, and rain, it’s designed to handle dynamic expansion and contraction across façades, curtain walls, and precast joints. Its non-sag formulation ensures clean vertical application, and it adheres strongly—often without primer—to both porous and non-porous substrates including glass, aluminium, masonry, and galvanised steel. Trusted by façade professionals, Dowsil 791 performs across Australia’s harshest climate zones with a wide service temperature range from -50°C to +150°C.
Core Features & Specifications
- ±50% movement capability ideal for high-performance façades and seismic joints
- Excellent weather resistance: UV, rain, ozone, and snow exposure
- Low VOC content (45 g/L) – compliant with Green Star IEQ-13 and air quality standards
- Strong unprimed adhesion to a variety of substrates, including aluminium, glass, and masonry
- Neutral-cure formulation suitable for use on coated or galvanised metals without corrosion
- Wide service temperature range: -50°C to +150°C
- Tack-free in 45 minutes, with 20-minute tooling time for precise finishing
The Dowsil 791 Silicone Weatherproofing Sealant Advantage
- Outperforms standard silicones with Class 50 joint movement under ASTM C920
- Ideal for demanding architectural and commercial façade applications
- Supports custom colour matching to blend with architectural surfaces
- Backed by Dow’s global warranty and technical documentation
- Available in 300ml cartridge and 500ml sausage formats for site flexibility
±50%
Joint Movement Capability – ASTM C920 Class 50
Best Practices: Do's & Don'ts
Do
- Use backing rod or bond breaker tape to control joint geometry
- Perform adhesion tests on porous or unusual substrates
- Tool within 15 minutes of gunning for clean, professional finish
Don't
- Do not apply on wet or frost-covered substrates
- Avoid painting over cured sealant – paint will not adhere
- Do not use where immersion or total confinement prevents moisture curing
Applications in Practice: Trade Scenarios
For Glaziers (On-site Applicator): Curtain Wall & IGU Joints
The Engineering Challenge
IGUs in curtain walls experience large thermal movements and require a sealant that adheres well to coated glass and aluminium without corrosion or primer.
The System-Based Solution with Dowsil 791
Dowsil 791 delivers strong adhesion to coated glass and aluminium with ±50% movement capacity, reducing call-backs and ensuring leak-free joints under daily expansion and contraction.
Best Practice Note
Apply using pneumatic gun for consistent bead. Tool within 15 minutes and remove masking tape immediately after finishing.
For Commercial Builders (Specifier/Architect): Precast Panel Sealing
The Engineering Challenge
Movement between precast concrete panels can result in joint cracking or water ingress if the sealant is not flexible and durable across seasons.
The System-Based Solution with Dowsil 791
With a Shore A hardness of 25–30 and resistance to UV and temperature extremes, Dowsil 791 ensures long-lasting performance on masonry and precast joints. Primer may be used for high-risk substrates.
Best Practice Note
Mock-up tests for stain resistance on natural stone and adherence to ASTM C1248 are recommended before full-scale application.
Performance Benchmark: Dowsil 791 vs. Standard Alternatives
| Metric | Dowsil 791 | Standard Silicone | Advantage |
|---|---|---|---|
| Joint Movement | ±50% High Flexibility | ±25% | Better for dynamic or seismic joints |
| Service Temperature | -50°C to +150°C Extreme Range | -30°C to +100°C | Suitable for all Australian climates |
| VOC Content | 45 g/L Low VOC | 150+ g/L | Better for Green Star projects |
| Adhesion Prep | Often primerless Faster Install | Primer required | Saves time and reduces labour |
Frequently Asked Questions
What is the joint movement capability of Dowsil 791 Silicone Weatherproofing Sealant?
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.
Can Dowsil 791 be applied to porous surfaces like precast concrete?
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.
What is the recommended tooling and working time?
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.
Is Dowsil 791 suitable for use in submerged or below-grade applications?
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.
Can Dowsil 791 be painted over after curing?
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.
Is Dowsil 791 compliant with Australian or international standards?
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.
Applications & Benefits
| 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 |
Applying Dowsil 791 Silicone Weatherproofing Sealant: Enhanced Protocols & Best Practices
-
Step 1: Surface Preparation
Ensure all substrates are clean, dry, and free from contaminants such as oil, dust, release agents, and old sealants.
Solvent Cleaning
Wipe surfaces with isopropyl alcohol or a recommended non-oily solvent using a lint-free cloth. Replace cloths frequently to avoid redepositing contaminants.
Mechanical Cleaning
Use wire brushing or light abrasion to remove laitance or loose particles from masonry or oxidised metals. Blow out joints with compressed air.
Timing: Clean immediately before application to avoid recontamination.Pro Tip: Use masking tape to protect edges and remove it immediately after tooling for crisp lines. -
Step 2: Joint Design & Backing
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.
Closed-Cell Backer Rod
Use non-absorptive, closed-cell rod to prevent bond to the back surface of the joint and allow movement accommodation.
Bond-Breaker Tape
In shallow joints where rod isn't possible, apply bond-breaker tape to the base to prevent three-sided bonding.
Timing: Apply backing immediately after substrate prep and before sealant application.Pro Tip: Use a depth gauge to verify uniform joint depth and avoid premature failure from overfilling. -
Step 3: Sealant Application
Apply Dowsil 791 using a caulking gun in a steady, continuous bead. Ensure full contact with both sides of the joint and backer rod.
Manual Cartridge Application
Use a standard or pneumatic caulking gun compatible with 300ml cartridges. Apply even pressure and avoid gaps or air bubbles.
Sausage Application
Use a barrel gun to apply 500ml sausages for high-volume jobs. Adjust nozzle to suit joint width and angle for best results.
Timing: Tool within 15 minutes before skin forms for optimal adhesion.Pro Tip: Work in shaded areas when possible. Higher temperatures reduce open time dramatically. -
Step 4: Tooling & Curing
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.
Manual Tooling
Use a rounded spatula or jointing tool to shape the bead. Do not add water or solvents during tooling.
Curing Conditions
Typical full cure takes 7–14 days at 25°C and 50% RH. Avoid water exposure during first 24 hours.
Timing: Tool immediately and allow curing undisturbed for optimal bond strength.Pro Tip: For multi-storey jobs, consider sequencing from top down to protect lower joints during curing.
Limitations & Safety
Product Limitations
Not suitable for structural glazing or load-bearing applications.
Do not use in submerged or below-grade joints—sealant requires atmospheric moisture to cure.
Not recommended for application on surfaces that bleed oils or plasticisers, including green rubber gaskets or uncured sealants.
Should not be painted over—paint will not adhere to cured silicone.
Use on natural stone only after passing ASTM C1248 stain testing. Staining may occur without proper testing and primer.
Consider Dow Stone Primer where needed
Safety & Handling Requirements
Personal Protective Equipment
Wear chemically resistant gloves (e.g., nitrile), safety glasses with side shields, and long sleeves to minimise skin contact. Avoid handling without protection.
Ventilation Requirements
Ensure adequate ventilation during application and curing, especially in enclosed or poorly ventilated areas. Avoid breathing vapours directly.
Fire & Handling Precautions
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.
Emergency First Aid
Eyes:
Flush immediately with clean running water for at least 15 minutes. Seek medical attention.
Skin:
Wipe off excess material, then wash thoroughly with soap and water. Remove contaminated clothing.
Inhaled:
Move person to fresh air. If symptoms persist, seek medical assistance.
Swallowed:
DO NOT induce vomiting. Rinse mouth with water and seek immediate medical help.
Seek medical attention immediately in all cases of serious exposure.
Dowsil 791 Silicone Weatherproofing Sealant Application Guide
Glass (Coated / Uncoated)
Surface Preparation
Clean glass thoroughly using a non-oily solvent and lint-free cloth. Ensure surface is dry and free from dust, grease, and protective coatings.
Primer Requirements
Primer typically not required for clean glass
Adhesion testing recommended for coated or reflective glass types.
Anodised or Powder-Coated Aluminium
Surface Preparation
Wipe with isopropyl alcohol or an appropriate cleaner to remove oily residues and dust. Use clean cloths to prevent cross-contamination.
No primer typically required
Ensure surfaces are fully cured and not chalking. Test adhesion on unknown coatings.
Precast Concrete & Masonry
Surface Preparation
Ensure surface is dry and cured for minimum 28 days. Remove laitance, dust, or loose particles using wire brushing or compressed air.
Primer recommended for critical applications
Dow primer may improve adhesion where surface conditions vary. Always test for staining on natural stone.
Dow 1200 OS PrimerGalvanised Steel
Surface Preparation
Wipe with solvent and dry cloth to remove zinc salts or oils. Slight abrasion may be used to expose fresh metal if corroded.
Primer not required for clean galvanised steel
Ensure compatibility with surface treatments or coatings.
Natural Stone (e.g. Granite, Marble, Limestone)
Surface Preparation
Clean using a pH-neutral cleaner and allow to fully dry. Avoid use on freshly honed or unsealed stone.
Primer required and stain testing mandatory
Sealant should pass ASTM C1248 stain testing before full application. Use only approved primers.
Dow Stone Primer (as tested)
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