DOWSIL 791 vs DOWSIL 795: which one for the job

Both are Dow silicones rated to 50% movement, and Cranco carries both. Dow does not treat them as interchangeable.

DOWSIL 791 is a weatherproofing sealant and Dow's own sheet rules it out of structural glazing. DOWSIL 795 is the sealant Dow lists for that work.

Which to take

Take DOWSIL 795 for glazing. Dow lists structural and nonstructural glazing, panel systems, panel stiffener and weather sealing, and its substrate list runs to glass, aluminum, steel, painted metal, EIFS, granite and other stone, concrete, brick and plastics.

Take DOWSIL 791 for weather sealing a facade. Dow lists general glazing and weather sealing in curtain wall and building facades, and expansion, connection, perimeter and movement joints. Its sheet says plainly that it is not for structural or insulated glazing.

On speed the two split further than the movement rating suggests. DOWSIL 791 is tack-free in 30 minutes and DOWSIL 795 takes 3 hours. Neither is paintable.

What the data sheets say

DOWSIL 791 against DOWSIL 795, from the figures each manufacturer publishes.
DOWSIL 791DOWSIL 795
ChemistrySiliconeSilicone
Movement capability±50 %±50 percent
Skin time20 minutes15 minutes
Tack-free time30 minutes3 hours
Cure time2.0 mm at 1 day, 4.0 mm at 3 days (23 °C, 50% RH)7 to 14 days curing (25 °C, 50% RH); full adhesion 14 to 21 days
Usesgeneral glazing and weather sealing in curtain wall and building facades; expansion, connection, perimeter, movement jointsstructural and nonstructural glazing, panel systems, panel stiffener, weather sealing
Package sizes310 mL cartridge; 600 mL sausage10.3 fl oz (305 mL) cartridge; 20 fl oz (590 mL) sausage; 2 and 4.5 gal (7.5 and 17 L) bulk
StandardsSNJF F&G-25E; ISO 11600-F&G-25LM; DIN 18540 Class F; EN 15651 CE markTT-S 001543A Class A; TT-S-00230C Class A; ASTM C920 Type S Grade NS Class 50 Use NT G A O; ASTM C1184; CAN2-19.13-M82

Which DOWSIL sealant is for structural glazing?

DOWSIL 795. Dow lists structural and nonstructural glazing among its uses, its own data sheet gives ASTM C1184 among its standards, and it says in the same breath that DOWSIL 791 is not recommended for structural glazing or insulated glazing.

What the manufacturers warn about

Do not use DOWSIL™ 791 Silicone Weatherproofing Sealant on bituminous substrates, substrates based on natural rubber, choloroprene or EPDM or on building materials and flexible plastics which might bleed oils, plasticizers or solvents. Do not use DOWSIL™ 791 Silicone Weatherproofing Sealant in a totally confined space because the sealant requires atmospheric moisture to cure. DOWSIL™ 791 Silicone Weatherproofing Sealant is not recommended for use on submerged joints or in joints where physical abuse or abrasion is likely to occur. Bleeding can occur on porous substrates such as concrete, marble, granite or other natural stones. On sensitive substrates, specific testing should be carried out. This product is not suitable for areas where food contact is likely. DOWSIL™ 791 Silicone Weatherproofing Sealant is not recommended for structural glazing or insulated glazing applications. This product is neither tested nor represented as suitable for medical or pharmaceutical uses.

Dow, on DOWSIL 791

DOWSIL™ 795 Silicone Building Sealant should not be used: In structural applications without prior review and approval by your local sales application engineer; In below grade applications; When surface temperatures exceed 50°C (122°F) during installation; On surfaces that are continuously immersed in water; On building materials that bleed oils, plasticizers or solvents that may affect adhesion; On frost laden or wet surfaces; In totally confined joints (the sealant requires atmospheric moisture for cure); If the sealant is intended to be painted (paints do not typically adhere to most silicone sealants); To surfaces in direct contact with food or other food-grade applications

Dow, on DOWSIL 795

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