What is butyl sealant or non-setting mastic used for?

Most sealants cure into an elastic solid that stretches and recovers. Butyl does something different: it never sets, so instead of stretching it simply flows and re-wets the surfaces as they move. That makes it superb in a compressed joint and useless as an exposed bead, because it stays soft enough to pick up dirt and to be pushed out of the joint.

Step by step

1. Understand where the seal comes from.A non-setting mastic seals by being trapped and compressed between two surfaces. It never depends on adhesion under tension, which is exactly why it works in a lap joint that flexes and why it fails as a bead in an open joint.
2. Use it for bedding and lapping, not for a visible bead.Bedding a glazing pane or a rooflight, sealing overlapping sheets of metal or plastic roofing, sealing a gutter union under a clip, bedding a flashing. In every case something clamps it in place.
3. Know the two forms.Gun-grade butyl in a cartridge for filling and bedding, and butyl tape or strip, which is a preformed compressible band. Tape is far cleaner to use and is standard for lap joints in sheet materials.
4. Compress it properly.The joint must be clamped, screwed or clipped so the mastic is squeezed out to a thin continuous film. An uncompressed butyl joint is not a joint at all — it is a lump of soft material sitting in a gap.
5. Do not use it where it is exposed.It never hardens, so it collects dust and grit, it can be smeared by a passing hand, and ultraviolet degrades the surface of some grades over time. If a joint will be seen or touched, use a curing sealant.
6. Do not paint it.Paint will not dry properly on a non-setting mastic and will craze and remain soft. If the joint has to be painted, it is the wrong product.
7. Expect it to stay removable, which is often the point.A gutter union bedded on butyl can be taken apart and rebedded years later. That demountability is a genuine advantage over a cured sealant, and it is why roofing and glazing use it.
8. Be prepared for the mess.It is extremely sticky, it does not wash off with water, and it stains. Wear gloves, keep white spirit and rags to hand, and mask anything you do not want it on.

Tips

• Butyl remains workable in the cold far better than most curing sealants, which is one reason roofers rely on it in winter.
• It will not stick to a dusty or greasy surface any better than anything else. Clean the joint faces first.
• White spirit takes it off tools and hands. Water and detergent do nothing.
• It stains porous surfaces such as brick and stone. Mask before you start rather than cleaning afterwards.
• Old butyl in a joint being remade should be scraped out. It never cures, so it stays as a contaminated layer that will not bond to anything new.

Alternatives

• Hybrid MS polymer: Cures into a flexible rubber, bonds to damp surfaces, resists ultraviolet and takes paint. The better choice for an exposed joint that must be both sealed and presentable.
• Silicone: Cures, waterproof, tools to a fine finish, and better in a permanently wet exposed joint. Cannot be painted and needs two-sided adhesion to work.
• Butyl tape: The preformed version of the same material. Much cleaner to apply than gun grade for lapped sheet joints, and easier to get a consistent thickness.
• EPDM or gasket seal: For a joint that is repeatedly opened, a proper gasket outlasts any mastic and does not need cleaning off each time.

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