The tape carries a resin that cures on contact with water, and the wrap becomes a rigid composite shell around the pipe.

The bandage is supplied in a sealed foil pouch to keep moisture out. Once opened, the tape is immersed or wetted, which starts the cure, and it is then wound around the prepared pipe under firm tension with a generous overlap on each turn. As it cures the resin bonds the layers to each other and to the pipe surface, producing a hard, load-bearing casing.

Cure begins in minutes and reaches handling strength quickly, with full strength taking longer. That short working time is the main practical constraint: everything - cleaning, any leak sealing, the pipe surface, tools and gloves - must be ready before the pouch is opened.

Most systems are used with a putty or sealing stick pressed into the defect first, so the bandage is providing strength over a plugged hole rather than trying to seal a jet by itself.

Where the shape is not a plain cylinder, where access is restricted, or where the defect is longer than a clamp will span.

Clamps are made for round pipe within a diameter range. They cannot seal on an elbow, a tee, a flange neck, a valve body, a reducer or a transition between sizes. A bandage conforms to any of those, because it takes the shape of whatever it is wound around.

Access is the second case. Fitting a clamp needs room to pass the band right around the pipe and to reach the bolts, which a pipe close to a wall, in a bundle or against a structure may not allow. A bandage needs only enough room to pass the roll around.

Third, a long split can be covered by continuing to wrap.

A clamp remains preferable on plain pipe with good access - it is faster, its rating is higher, and it does not depend on surface preparation.

Clean, sound, dry and roughened - and this is where most bandage repairs fail.

The repair works by bonding to the pipe. Loose rust, scale, flaking paint, grease, oil and wet mud all prevent that bond, and a bandage applied over them makes a shell that grips nothing and slides under pressure.

Wire brush or abrade the pipe back to a sound surface, extending well beyond the defect in both directions to give the wrap a bonding length. Roughen a smooth or painted surface deliberately so the resin has a key. Degrease if there is any oil. Dry the surface as far as practicable.

An active leak must be stopped or substantially reduced first, usually with a sealing putty pressed into the defect, because resin cannot cure properly with water flowing through it. Depressurising the line, even briefly, makes an enormous difference to the quality of the result.

Usually not, and it should be recorded as temporary or semi-permanent unless the manufacturer explicitly rates it otherwise for that duty.

The cured composite is strong, but its pressure and temperature ratings are generally below those of a mechanical clamp, and its performance depends heavily on how well it was applied - surface preparation, tension, overlap and cure conditions all vary with the person and the circumstances in a way a bolted clamp does not.

It is also difficult to inspect. A cured wrap conceals the pipe, so deterioration underneath is invisible until it fails.

Use it for what it is excellent at: getting a plant back on line, stopping a leak on a shape nothing else fits, and buying time until a shutdown. Then log it with the date, the location, what was wrapped and why, and programme the permanent repair. Bandages that are forgotten become the failures nobody could explain.

Moderate ones, and always as a combination of the two together with the pipe size.

The cured composite's capability falls as temperature rises, because the resin softens. It also depends on the number of layers applied and on the pipe diameter, since hoop stress in the wrap rises with diameter for a given internal pressure.

Manufacturers publish tables giving the number of layers required for a given pipe size, pressure and temperature, and these should be followed rather than estimated - 'a few more turns for luck' is not the same as the specified layer count, and neither is stopping early because the roll ran out.

Buy enough bandage for the calculated wrap before starting. Because the cure begins when the pouch is opened, running short part way through a wrap means the repair is compromised and there is no opportunity to correct it.

On hot pipework within the resin's temperature limit, and on plastic pipe with attention to bonding and expansion.

Hot service is common - steam and hot water lines are frequent candidates - but the resin has a maximum service temperature and cure behaves differently on a hot surface, generally accelerating so working time is even shorter. Some systems have specific hot-application grades. Check both the cure instructions and the service rating.

Plastic pipe bonds less readily than steel, so abrading the surface to give a mechanical key matters more, and some systems specify a primer. Plastic also expands and contracts far more than the composite shell does, so a wrap on a line that cycles in temperature is working against differential movement.

For PVC in particular, remember that a crack propagates. A wrap may hold the leak while the crack extends beyond the wrap, so cutting out the section is usually the sounder decision.

Bandage sizes matched to the pipework actually installed, plus the sealing putty, abrasive and gloves - stored where the pouches stay sealed and dry.

The wrap is only useful if it is to hand when the leak happens, and the shelf life depends on the foil pouch remaining intact. A punctured pouch cures in storage and the roll is scrap, so check stock periodically rather than assuming it is serviceable.

Stock the sizes that fit the site's pipework rather than a single general size, and include the sealing putty - a bandage without something to plug the hole is much less effective.

Gloves are not optional. Uncured resin bonds readily to skin and is an irritant, and the cure generates heat.

Most usefully, make sure someone has applied one before. The working time is short, and reading the instructions for the first time with water spraying is not the moment to learn the technique.