Sealant Extrusion And Placement
Sealant should be gun-applied in a continuous flow to provide a joint without gaps or voids. This will prevent the entry of both condensation from above and water drawn up by capillary action, either of which can cause metal corrosion.
After placing the sealant cartridge in the gun, the tapered nozzle should be cut with a sharp knife at an angle of 45 ° at a position which would provide a bead of sealant approximately 6 mm in diameter.
Sealed rivets are the preferred fastener. The stem holes of unsealed blind rivets must be sealed with silicone, which can attract dust and look unsightly.
Sealant should be placed in a continuous bead close to the weather end, covering any pre-drilled rivet holes. If a complete ring of sealant is applied around the fastener, an air pocket forms which compresses during tightening of fasteners. It can blow a channel, thus preventing an effective seal.
The best method for lap fabrication is.
- Assemble and drill the components. Rivets should be positioned at 50 mm spacings.
- Separate the components and remove any drilling swarf.
- Remove any strippable film.
- Clean the joint.
- Apply a continuous sealant bead as described above.
- Relocate the components, align the holes and fasten.
- Externally seal the centre of all rivets if required.
- Remove surplus sealant after it has cured (not while uncured), to avoid smearing.
When placing the overlapping sheet, care should be taken to avoid disturbing the sealant when fastening.
Excess sealant should be removed with a plastic spatula or purpose made plastic scoop as excess sealant collects dirt that is unsightly.
Visible excess of sealant is not an acceptable trade practice.
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