Metal repair epoxy putty: how to choose a metal-filled or ceramic-filled epoxy for cold repairs

What a metal-filled (cold weld) epoxy is

A metal repair epoxy putty is a two-component epoxy — resin and hardener — loaded with steel, stainless steel, aluminum or ceramic particles. Choosing one means matching the filler, the physical form and the resin system to the substrate, the service and the repair, then preparing the surface properly: preparation decides adhesion more than the product does.

The resin system sets chemical resistance, temperature limit, cure behaviour and the ability to wet a damp surface. The filler sets the body of the product and its cured behaviour: machinability, hardness, abrasion resistance, thermal expansion closer to that of the metal, colour. Curing is a chemical reaction, not drying, so the mix ratio on the data sheet is a requirement, not a suggestion.

"Cold weld epoxy" is a marketing name for the same family. Nothing is welded: the repair is an adhesive bond to a prepared surface, with the strengths and weaknesses of an adhesive joint. Whether to bond or weld at all is covered in cold bonding vs welding; this guide starts after that decision.

Paste, pourable or brushable: forms of metal repair epoxy

The same chemistry comes in several consistencies; the form follows the geometry of the repair.

Machine chocking and baseplate grouting use a separate category, epoxy grout.

Choosing the filler: steel, stainless, aluminum or ceramic

Two neighbouring families come up in the same selection: elastomeric compounds, which tolerate impact and flexing better, and novolac epoxy grades, formulated for higher temperatures and harsher chemicals.

How to choose a metal-filled epoxy: selection criteria

Substrate and geometry

Start with the base metal. Cast iron from oil service holds oil in its pores and must be treated first. Then the geometry: a pitted area takes a paste, a deep cavity a pourable grade behind a dam, a large worn surface a rebuild paste followed by a brushable coating.

Service conditions: wet, immersion, temperature, chemicals

• Dry or immersion. The data sheet gives separate temperature limits for dry service and for immersion, the latter lower. Immersed repairs need a pinhole-free film, verified by holiday test where required.
• Chemicals. Fluid, concentration, temperature and continuous or occasional contact are checked against the manufacturer's chemical resistance chart before selection.
• Potable water. The product must be certified for drinking water contact, typically to NSF/ANSI/CAN 61.

Abrasion, erosion and cavitation

Fine particles sliding along a surface call for a ceramic-filled grade; coarse particles striking at steep angles break brittle coatings and call for elastomeric linings or ceramic tile. Where erosion-corrosion is at work, the repair needs both the rebuild and an erosion-resistant top layer. Cavitation damage is treated the same way, but the coating does not change the hydraulics, as detailed in centrifugal pump repair with epoxy.

Thickness to rebuild

The reaction gives off heat and a thick mass heats itself, so data sheets set a maximum thickness per application. Deep losses are rebuilt in layers within the recoat window, or with inserts or a backing plate that reduce the volume of compound. A through-hole is bridged with a backing, not plugged with putty.

Machinability and finishing

If the repaired area has to be bored, faced, drilled or tapped, choose a metal-filled grade and machine it after full cure. Ceramic-filled grades are formed to dimension during application — templates, release-coated mandrels, straightedges — or ground. Seat bores in valves follow this logic: see valve body repair with epoxy. Gasket faces are rebuilt and formed flat, as in flange face corrosion repair without machining.

Cure time and temperature

Pot life shortens as temperature rises and as the mixed mass grows. Cure slows as temperature falls, and below the product's minimum application temperature it may not complete. The data sheet gives times to handling, machining and full service at several temperatures; read them at the actual temperature. In a cold plant, heat the part, not only the air, and keep the surface above the dew point by the margin the specification requires.

Wet, oily or leaking surfaces

Standard grades need a dry, clean surface. Specific grades tolerate damp surfaces or are made for emergency sealing of low-pressure leaks; they do not make preparation optional. An emergency leak seal is usually temporary until a planned repair, as discussed in pipe leak repair without shutdown.

Surface preparation: the deciding factor

Most failed epoxy repairs let go at the interface: a preparation problem, not a product problem. The data sheet sets the level.

1. Degrease first. Remove oil and grease by solvent or detergent cleaning (SSPC-SP 1) before any abrasive work, or blasting drives contamination into the surface. Oil-soaked cast iron is heated to draw oil out of the pores, then cleaned again.
2. Remove soluble contaminants. Chlorides and other soluble salts left on corroded steel cause blistering under the repair. Test per ISO 8502-6 and ISO 8502-9 and wash until the specified limit is reached.
3. Abrade to bare metal with a profile. Blasting to near-white metal (ISO 8501-1 Sa 2½ / SSPC-SP 10) is the usual requirement; where blasting is impossible, the data sheet may accept power-tool cleaning to bare metal (SSPC-SP 11). The surface profile, measured per ISO 8503, is what the compound keys into. A surface ground shiny or wire-brushed has no usable profile.
4. Clean and apply promptly. Remove dust (ISO 8502-3) and apply before flash rust forms, pressing the first layer firmly into the substrate to wet it.

The standards behind these steps are compared in the surface preparation guide (NACE No. 2 / SSPC-SP 10).

Common mistakes

• Grinding to a bright finish and calling it prepared: no profile, poor adhesion.
• Splitting a kit by eye instead of measuring to the stated ratio, or mixing a batch too large for the pot life.
• Building too thick in one pass, or overcoating after the recoat window without abrading.
• Using a ceramic grade where the area must be machined, or a steel grade as the top layer on an erosion surface.
• Returning to service in the cold on the room-temperature cure time.
• Feathering the edges to nothing on surfaces exposed to flow or impact, where they lift; key the edges into a step or groove.

Limits: what a filled epoxy does not do

• Pressure boundary. The epoxy is not counted as pressure-retaining wall. When the remaining wall is below the minimum or the defect is through-wall, the decision belongs to an engineering assessment under the governing code — see fitness-for-service. Engineered composite repairs designed with calculations under ASME PCC-2 or ISO 24817 are a different system, covered in ASME PCC-2 composite repair.
• Temperature. Above the data sheet limit, the polymer softens and loses adhesion.
• Load type. An adhesive joint is strongest in compression and shear, weakest in peel and cleavage; point loads, impact and bending at the edge shorten its life. Metal-to-metal seats and heavily loaded sliding contacts are machined, hardfaced or replaced.
• Root cause. A coating does not correct cavitation, misalignment or an undersized component.

Industrial filled epoxy vs consumer "metal glue"

Consumer cold weld syringes and sticks share the chemistry and suit small, dry, lightly loaded repairs. The difference is documentation. An industrial repair needs a technical data sheet stating mix ratio, pot life and cure schedule by temperature, service temperature dry and in immersion, chemical resistance, surface preparation and thickness per layer, plus a safety data sheet. Without them, the product cannot be matched to the service. Instant adhesives sold as metal glue do not fill gaps and are not suited to rebuilds.

Selection table by type of repair

FAQ

Is a metal-filled epoxy as strong as a weld? No direct comparison applies. A qualified weld becomes part of the metal; an epoxy repair is an adhesive joint whose strength values are in the data sheet, and it is designed to work in compression and shear.

Can metal repair epoxy be drilled and tapped? Metal-filled grades can, after full cure. For heavily loaded fasteners, a threaded insert set in the compound is the usual approach. Ceramic-filled grades are not machined conventionally.

Will epoxy putty stick to a wet or oily surface? Standard grades will not. Wet-surface grades exist for damp conditions and emergencies, but oil must still be removed and the surface abraded.

How long before the equipment can return to service? It depends on the product and the temperature. The data sheet gives times to light load, machining and full immersion service; in the cold, they lengthen.

Is "cold weld" different from metal-filled epoxy? No. It is a trade name for a filled epoxy repair compound; performance depends on resin, filler and preparation.

In practice

Selecting the grade is half of a cold repair; the other half is preparation, application and cure. Induscoat carries out cold bonding repairs with metal- and ceramic-filled epoxies — assessment, product selection against the data sheets, surface preparation, rebuild, machining or forming, protective coating and inspection — as part of its maintenance and rehabilitation service for plants in Quebec and Ontario. Photos of the damage, the base metal, the fluid and the temperature are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.