Which Plastics Epoxy Can Bond—and How to Avoid a Failed Repair

The short answer: epoxy sticks to some plastics, not all

Yes, two-part epoxy can bond some plastics—but “plastic” is too broad a category to predict whether a repair will hold. Adhesion depends on the polymer, the epoxy formulation, surface condition and preparation, curing process, joint demands, and service environment.

An epoxy adhesive normally consists of resin and hardener that must be combined before application. Mixing starts the curing reaction. Because formulations differ, compatibility claimed for one plastic epoxy does not automatically apply to every product sold as epoxy. The current product label, technical data sheet, substrate-compatibility chart, and application instructions should control product selection and use.

As a conservative screening guide based mainly on commercial technical guidance rather than standardized independent testing:

• Rigid PVC and ABS are comparatively promising when they are clean, dry, suitably prepared, and paired with a compatible epoxy.
• Polyethylene (PE), polypropylene (PP), and PTFE are poor candidates for ordinary epoxy.
• Acrylic, polycarbonate, nylon, and polystyrene are conditional candidates. Some plastic-specific epoxies list them, but performance may depend on the grade, preparation method, stress in the part, epoxy formulation, and service conditions.

WEST SYSTEM, for example, identifies rigid PVC, ABS, and styrene as plastics on which better adhesion may be obtained with preparation and sufficient bonding area, while recommending an adhesion test when behavior is uncertain. This is manufacturer guidance, not a cross-product strength ranking. See WEST SYSTEM’s plastic surface-preparation guidance.

Before committing to a repair:

1. Identify the plastic.
2. Check whether the selected epoxy specifically names that polymer.
3. Assess whether the part flexes, carries substantial load, or could cause harm if it fails.
4. Prepare a representative test bond on scrap or a hidden area.
5. Evaluate the test only after the manufacturer’s stated full cure.

Do not assume epoxy is universally the strongest glue for plastic, stronger than the original part, waterproof in every formulation, or suitable for structural, outdoor, high-temperature, or safety-critical service. Those properties are product- and application-specific.

Joint geometry is another practical consideration, although the supplied commercial sources do not provide validated design rules. For any consequential joint, household testing and general compatibility guidance are not substitutes for application-specific engineering data.

Identify the plastic before choosing the adhesive

Look for an embossed polymer abbreviation or recycling mark before judging the plastic by appearance. The mark may be inside a container, on the back of a housing, under a cap, or near an injection-moulding date code.

Code	Common abbreviation	Plastic family	Immediate epoxy implication
#1	PET or PETE	Polyethylene terephthalate	Check the epoxy manufacturer’s compatibility information
#2	HDPE or PE-HD	High-density polyethylene	Poor candidate for ordinary epoxy
#3	PVC	Polyvinyl chloride	Comparatively promising when rigid and properly prepared
#4	LDPE or PE-LD	Low-density polyethylene	Poor candidate for ordinary epoxy
#5	PP	Polypropylene	Usually needs a PP-rated system or another repair method
#6	PS	Polystyrene	Conditional; verify the exact adhesive and preparation
#7	Other	Assorted plastics	The polymer remains uncertain; investigate and test

A specialty adhesive retailer associates these codes with PET, HDPE, PVC, LDPE, polypropylene, polystyrene, and assorted plastics respectively, while cautioning that adhesive selection still depends on the individual polymer. See the recycling-code and adhesive overview from Gluegun.com.

The most important distinctions for an epoxy repair are:

• #2 and #4 are forms of polyethylene.
• #3 is PVC, which is often more receptive to a compatible epoxy.
• #5 is polypropylene, another difficult low-surface-energy plastic.
• #6 is polystyrene, for which product compatibility should be checked.
• #7 does not identify one polymer.

A #7 item might be polycarbonate, acrylic, a blend, a composite, or another material outside the first six categories. The number alone cannot establish epoxy compatibility.

Even a specific recycling mark does not reveal everything relevant to bonding. It may not identify the exact grade, fillers, reinforcement, plasticizers, pigments, paint, plating, surface treatment, weathering, or mould-release residue. Two parts carrying the same broad polymer abbreviation may therefore respond differently to the same adhesive and preparation process.

For an unmarked item, check its manual, packaging, replacement-part listing, or manufacturer documentation. Ask for a polymer designation or chemical-resistance table where available. Appearance and feel are weak evidence because unrelated plastics can all be clear, glossy, waxy, rigid, or flexible.

If identity remains uncertain, test a spare moulding tab, offcut, hidden flange, or another representative piece made from the same material. A test on an unrelated plastic that merely looks similar provides little information about the intended repair.

Plastic-by-plastic epoxy compatibility guide

The following table is a conservative screening tool, not proof of bond strength, service life, or safety. Its ratings summarize recurring manufacturer and retailer guidance; the supplied evidence does not include standardized independent comparisons across polymers and epoxy products. Product-specific compatibility claims should be checked in the adhesive’s current technical documentation. Loctite, for example, lists ABS, PVC, acrylic, nylon, polycarbonate, and fiberglass-reinforced plastic for one particular plastic epoxy, but that claim cannot be generalized to every formulation. Review Loctite’s compatibility and application guidance.

Plastic	Identification clue	Screening assessment for ordinary epoxy	Preparation direction	When to consider an alternative
Rigid PVC	PVC, vinyl, or #3	Comparatively promising with a compatible product	Clean safely, dry fully, and abrade only if the plastic and adhesive instructions permit it	Consider a dedicated PVC system when it better fits the joint
ABS	ABS marking; sometimes grouped under #7	Comparatively promising	Remove contamination, abrade lightly if specified, remove debris, and bond promptly	Consider a dedicated plastic adhesive, welding, or replacement for thin, flexible, or highly stressed parts
Acrylic (PMMA)	PMMA, acrylic, or sometimes #7	Conditional	Test the cleaner, preparation method, and epoxy on scrap or a hidden area	Consider an acrylic-specific system where appearance or clarity matters
Polycarbonate (PC)	PC or sometimes #7	Conditional	Use only a cleaner confirmed compatible with the plastic; abrade cautiously if permitted	Choose a PC-rated adhesive for clear, thin, stressed, or impact-exposed parts
Nylon (PA)	PA, PA6, PA66, or nylon	Conditional	Follow product-specific cleaning and abrasion instructions	Use a nylon-rated adhesive or specified pretreatment if ordinary epoxy is not approved
Polystyrene (PS)	PS or #6	Conditional	Use gentle, material-compatible preparation; exercise particular care with foam	Select a PS-rated adhesive if the epoxy does not expressly list it
HDPE	HDPE, PE-HD, or #2	Poor	Do not rely on sanding alone; follow only the process specified by a PE-rated system	Consider a low-surface-energy adhesive, suitable welding process, mechanical repair, or replacement
LDPE	LDPE, PE-LD, or #4	Poor	As for HDPE; visible roughness does not prove compatibility	Consider a specialty system, welding, mechanical repair, or replacement
Other polyethylene	PE marking	Poor	Follow a purpose-made process, including any specified primer or activation	Consider specialty adhesive, welding, mechanical retention, or replacement
Polypropylene	PP or #5	Poor	Use a system explicitly rated for PP and follow its primer or activation requirements	Consider PP-specific adhesive, welding, mechanical repair, or replacement
PTFE	PTFE marking	Exceptionally difficult	Do not treat routine sanding and household epoxy as a dependable process	Use a specialist industrial process or non-adhesive design
Silicone	Silicone or elastomer marking	Generally unsuitable for ordinary epoxy	Do not assume abrasion will create a dependable bond	Use a silicone-compatible specialist system, mechanical retention, redesign, or replacement
Flexible or plasticized material	Bends readily; may be flexible PVC, PE, PP, or an elastomer	Mechanically risky even if initial adhesion occurs	Verify both substrate compatibility and the flexibility of the cured adhesive	Consider a compatible flexible adhesive, welding, mechanical repair, or replacement

Rigid PVC and ABS are repeatedly presented as relatively favorable epoxy substrates, but that remains a screening conclusion. Plastic grade, additives, contamination, surface finish, adhesive formula, joint geometry, and exposure may all change the outcome.

Acrylic, polycarbonate, nylon, and polystyrene require more caution because commercial recommendations vary. Some plastic-specific epoxies name them; others do not. The preparation method also matters. A cleaner or solvent suitable for one polymer may alter another, so test the complete process—not just the epoxy—on scrap or an inconspicuous area.

HDPE, LDPE, other polyethylene grades, and polypropylene are poor candidates for ordinary epoxy because commercial technical guidance attributes their bonding difficulty to low surface energy and inadequate wetting. Master Bond directs users toward material-specific primers or controlled plasma or corona treatment for PE and PP rather than treating abrasion alone as sufficient. See Master Bond’s substrate-specific preparation guidance.

PTFE is more resistant to routine bonding still. A routine household sand-and-epoxy process should not be expected to produce a dependable PTFE joint.

Silicone is also generally unsuitable for ordinary epoxy under the available commercial guidance. That does not rule out every specialized industrial system, but it does mean that a general-purpose epoxy should not be selected merely because it hardens against the surface.

Flexibility creates a separate problem. A rigid epoxy may initially adhere to a flexible substrate yet crack or release as the part bends. The substrate and cured adhesive do not necessarily deform together, so a successful static test may not represent repeated service movement.

Environmental exposure may also affect suitability, but the supplied sources do not establish quantified durability for each plastic-and-epoxy combination. Water, temperature changes, sunlight, vibration, chemicals, impact, and sustained loading should be assessed against the selected product’s technical data rather than inferred from an initial hand test.

Why sanding helps some plastics but not PE, PP, or PTFE

For an adhesive to bond, the uncured material must make close contact with the substrate. In practical terms, it needs to wet the surface rather than retract, bead up, or sit on top of contamination. It must then develop useful interactions at the interface as it cures.

On a suitable rigid plastic, abrasion may:

• Remove weak or weathered surface material
• Dull a smooth or glossy surface
• Increase the available contact area
• Create texture into which cured epoxy can mechanically key

Mechanical keying is not the same as raising the plastic’s surface energy. This distinction explains why sanding may help compatible rigid plastics but is not a dependable standalone solution for polyethylene or polypropylene.

A sanded PE or PP surface can look dramatically rougher while retaining the low-surface-energy behavior that makes reliable wetting difficult. PTFE is more resistant still. Aggressive sanding should not be assumed to convert any of these materials into an ordinary epoxy substrate.

Contamination can also block contact with the underlying plastic. Potential problem layers identified in manufacturer guidance include:

• Oil and grease
• Wax or polish
• Silicone residue
• Mould-release agents
• Old adhesive
• Loose paint or coatings
• Moisture
• Sanding dust
• Residue from an unsuitable cleaner
• Oil or water deposited by compressed air

If epoxy cures against a weakly attached film, the repair may release with that film rather than remain attached to the plastic. This is why preparation requires more than making the surface visibly rough.

Extra adhesive thickness does not correct a weak interface. A gap-filling layer may rebuild a missing corner or occupy space between poorly fitting pieces, but the repair still depends on the thin boundary where epoxy first contacts the plastic. A later coat bonds to the previous epoxy layer, not directly to the hidden substrate.

Material-specific primers and controlled corona, plasma, or other activation processes may improve adhesion to certain difficult plastics. Their procedures are not interchangeable, and the effective bonding window can be product-specific. Master Bond notes that some surface treatments may have a short useful life and recommends treating close to bonding time. Review the manufacturer’s discussion of physical surface treatments.

Some technical guides also describe flame treatment or chemical etching. Those procedures can damage plastic and may involve substantial process hazards. Do not improvise them from an incomplete recipe. Use a complete manufacturer-specified system with appropriate equipment and controls, or choose a safer adhesive or repair method.

A low-risk surface-preparation process

Use this sequence:

Identify → inspect → clean → abrade if appropriate → remove debris → dry → bond promptly

1. Identify and inspect

Confirm the polymer as closely as possible. Then inspect the repair area for cracks, distortion, missing material, old adhesive, coatings, and signs that the plastic has become brittle or degraded.

Dry-fit the pieces before opening the epoxy. Confirm that they align without excessive force and that a clamp, tape, jig, or other fixture can hold them in position. If the adhesive must bridge a large unsupported gap or the pieces cannot be aligned reliably, reconsider the repair method.

Decide whether sanding itself could damage the item. Thin, soft, transparent, foamed, painted, plated, textured, or decorative plastic may need a gentler process or a different adhesive.

2. Remove loose material and contamination

Remove labels, loose coatings, failing adhesive, oil, wax, dust, silicone, and mould-release residue from the intended bond area. Do not assume that bonding over paint or plating creates a direct bond to the plastic; the repair would depend on that intermediate layer remaining attached.

Clean before sanding. Otherwise, abrasive paper may work oil, wax, silicone, or mould-release residue into the scratches and spread it over a larger area. WEST SYSTEM specifically recommends cleaning before abrasion and identifies grease, oil, wax, mould release, silicone, sanding residue, compressor oil, and water as potential adhesion problems. Its guidance also covers dust removal and contaminated compressed air.

3. Choose a plastic-compatible cleaner

Use only a cleaner supported for the exact polymer and finish. Check the plastic manufacturer’s chemical-resistance information, the adhesive technical data sheet, and the cleaner’s current safety data sheet. Test the cleaner on scrap or an inconspicuous area before treating a visible or important component.

Do not treat acetone, alcohol, lacquer thinner, or any other solvent as universally safe. Manufacturer guidance warns that the wrong adhesive or preparation chemical can damage plastic, while solvent-based products may alter or warp certain surfaces. Clear, thin, finished, foamed, or stressed parts deserve particular caution. Gorilla’s guide advises matching the adhesive and preparation process to the specific plastic.

Stop if the surface softens, swells, warps, becomes cloudy, loses its finish, or develops fine cracking. If compatibility cannot be established, do not experiment on an important visible or load-bearing area.

Follow the cleaner and adhesive labels for gloves, eye protection, ventilation, storage, and disposal. LePage’s application guidance, for example, calls for gloves, eye protection, and a ventilated workspace, but the exact precautions remain product-specific. See LePage’s epoxy application precautions.

4. Abrade only when appropriate

If both the plastic and adhesive guidance call for abrasion, scuff the bond area evenly. Aim for a uniformly dulled surface rather than deep grooves. Avoid thinning a wall, rounding an edge, enlarging a hole, overheating the plastic, or changing the fit of a close-tolerance joint.

There is no evidence-supported universal sandpaper grit for every plastic. Commercial recommendations range from relatively coarse abrasion to much finer scuffing because the appropriate finish depends on the polymer, adhesive, part thickness, and joint. Follow the selected product’s current instructions rather than averaging conflicting grit numbers.

If instructions are absent, do not conduct aggressive experiments on the final component. A representative sample can reveal whether the method gouges, smears, heats, clouds, or otherwise alters the material.

5. Remove debris without recontaminating the surface

Remove all loose sanding dust using a method that does not redeposit skin oil, water, compressor oil, dirty-rag residue, or incompatible cleaner residue. Depending on the product instructions, a clean brush, suitable vacuum method, or approved wipe process may be appropriate.

Avoid touching the prepared area. Hold the part outside the bond zone or use clean gloves.

6. Dry completely

Allow the part to dry fully before bonding. Watch for condensation when a cold component is brought into a warmer or more humid workspace. WEST SYSTEM specifically warns that changing temperature conditions can produce condensation and advises ensuring that bonding surfaces are dry.

Let the component reach a stable working temperature that falls within the epoxy’s permitted application range. Do not use improvised heating that could distort the plastic or exceed the adhesive’s limits.

7. Bond promptly

Once the surface is clean, prepared, and dry, apply the epoxy without unnecessary delay. Dust may settle, hands may touch the area, and activated surfaces may have a product-specific bonding window.

If a primer or activation process is required, follow the named system’s order of operations, application amount, drying conditions, and maximum time before bonding. A generic primer is not automatically interchangeable with the one specified by the adhesive manufacturer.

Mix, apply, fixture, and cure the epoxy correctly

The epoxy manufacturer’s current instructions are the controlling authority for:

• Substrate compatibility
• Resin-to-hardener ratio
• Mixing method and duration
• Working time
• Permitted bond thickness
• Application temperature
• Clamping or fixturing
• Initial set and handling time
• Full cure
• Service limits
• Personal protective equipment

Two-part epoxy depends on the specified ratio and thorough mixing. An inaccurate ratio or unmixed streaks can prevent the material from curing as intended.

A general workflow is:

1. Arrange the prepared parts, fixture, tools, and protective equipment before dispensing.
2. Dispense the specified amounts of resin and hardener onto a suitable disposable surface, unless the product uses a mixing nozzle.
3. Mix for the stated time and in the stated manner.
4. Apply the adhesive within its working period.
5. Bring the surfaces together in their final position.
6. Fixture the joint so the pieces cannot move.
7. Leave the repair undisturbed for the required cure period.

Some manufacturer guides recommend coating both surfaces. That may improve coverage for certain products and joints, but it is not a universal rule. Follow the instructions for the epoxy actually being used.

The fixture should control the assembly rather than crush it.

Do not confuse:

• Working time: the period during which mixed epoxy can be applied and assembled
• Initial set: the point at which it begins to become firm
• Handling time: when the assembly may tolerate limited movement
• Full cure: when the manufacturer states that the specified cured properties have developed

Fast-set wording is not permission to load the repair. Loctite’s application guidance distinguishes working and setting instructions from the need to wait for the manufacturer’s stated full cure, and it notes that warmth may accelerate curing while cold may slow it within the product’s permitted conditions. See Loctite’s product-specific mixing, clamping, and cure guidance.

Keep the repair free from load, water, impact, flexing, chemicals, and service temperatures until full cure has elapsed. Because cure schedules vary by formulation and temperature, use the timing on the current package or technical data sheet rather than a generic number.

Gel or gap-filling epoxy may be useful on vertical surfaces or for replacing a missing fragment in a compatible rigid plastic. Gap-filling ability does not overcome an incompatible substrate: a fully cured filler may still release from PE, PP, PTFE, silicone, or a contaminated surface.

When epoxy is the wrong repair method

There is no universal “best plastic glue.” Choose the repair method according to the polymer, rigidity, joint, exposure, and consequences of failure.

Situation	More suitable direction
HDPE, LDPE, or another PE	Purpose-made low-surface-energy adhesive with its specified primer or activation; suitable plastic welding; mechanical repair; or replacement
Polypropylene	PP-rated adhesive and specified primer or activation; suitable welding; mechanical attachment; or replacement
PTFE	Specialist industrial bonding process or non-adhesive retention
Silicone	Silicone-compatible specialist system, mechanical retention, redesign, or replacement
Repeatedly flexed part	Compatible flexible adhesive, suitable welding, mechanical repair, or replacement
Rigid PVC assembly	Compatible epoxy or, where appropriate, a dedicated PVC adhesive or cement
Acrylic assembly	Acrylic-specific system where appropriate, with careful material-compatibility checks
Compatible rigid plastic with a missing chip	Gel or gap-filling epoxy after substrate compatibility is established
Small or poorly fitting joint	Reconsider the geometry, use a suitable mechanical method, or replace the part
Consequential component	Application-specific validation or replacement rather than an unverified household repair

For PE and PP, use a bonding system that explicitly names the polymer and follow every specified primer or activation step. “For most plastics” is less useful than a technical data sheet that names HDPE, LDPE, PE, or PP. Do not substitute a generic primer or assume rough sanding performs the same function.

Plastic welding may be appropriate when a thermoplastic can be fused with compatible material and the part geometry permits the process. Simply melting a surface with an improvised heat source can deform the component and is not equivalent to a controlled welding procedure.

For repeatedly flexed parts, a rigid epoxy patch may be mechanically mismatched. A compatible flexible adhesive may accommodate movement better, but it must still be rated for the polymer and service environment.

Dedicated plastic or solvent-cement systems may be preferable for some PVC or acrylic assemblies. These products are not interchangeable across polymers, and solvent-based systems can alter plastic surfaces. Confirm compatibility in the product documentation and test the process before applying it to a visible or important part.

Do not treat a successful cosmetic repair as validation for:

• Pressure-containing components
• Highly loaded brackets
• Vehicle, lifting, or protective components
• Hot-fluid or high-temperature service
• Electrical safety enclosures
• Any part whose failure could cause injury or major damage

The supplied evidence does not provide standardized capacity or durability data for those applications. Where failure has serious consequences, replacement or application-specific professional validation is more controlled than relying on a household compatibility test.

For food-contact, potable-water, medical, or aquarium use, do not infer suitability from hardness, lack of odor, or apparent cure. Consult the product manufacturer’s current use limitations and the requirements applicable to the specific application.

Test the bond and troubleshoot common failures

When plastic identity or epoxy compatibility is uncertain, prepare a test bond on scrap or a hidden representative area. Reproduce the final process as closely as possible:

• Same plastic and surface condition
• Same cleaner
• Same abrasion method
• Same epoxy and mix procedure
• Similar adhesive thickness
• Same fixture method
• Same cure temperature
• Same full-cure duration

Evaluate the sample only after the manufacturer’s stated full cure. An early pull test may reject a bond before the curing process is complete. Loctite notes that cure times vary by adhesive and gives approximately 24 hours for one cited plastic epoxy; that timing is product-specific and must not be transferred to another formulation. Check the product-specific cure information in Loctite’s plastic-glue guide.

A representative test can screen for obvious incompatibility, but a hand pull does not establish numerical strength, aging resistance, environmental durability, or suitability for consequential service.

If the cured epoxy peels cleanly from the plastic

Clean release at the plastic-to-epoxy boundary suggests that the interface deserves review, but appearance alone is not a validated technical diagnosis. Check:

• Whether the plastic was identified correctly
• Whether it is PE, PP, PTFE, silicone, or another resistant material
• Whether the epoxy specifically lists the polymer
• Whether oil, wax, mould release, silicone, moisture, coating, or dust remained
• Whether the cleaner left residue or altered the surface
• Whether a specified primer or activation step was omitted
• Whether the part flexed during curing or service

Do not simply add more epoxy. A thicker mass does not change the original contact between the first layer and the plastic.

If the epoxy remains soft or tacky

Review the specified resin-to-hardener ratio, dispensing method, mixing thoroughness, substrate and room temperature, elapsed full-cure time, and current product instructions. Also consider whether the material was old, contaminated, or stored outside its stated conditions.

These are areas to investigate, not an exhaustive diagnosis. Do not place incompletely cured adhesive into service.

If the patch cracks or releases when the plastic bends

The cured adhesive may be too rigid for the substrate or movement of the joint. Reconsider a polymer-compatible flexible adhesive, controlled plastic welding, mechanical repair, or replacement.

A larger rigid patch does not necessarily eliminate the mismatch. Test the proposed alternative on representative material before relying on it.

If the plastic becomes warped, cloudy, soft, or crazed

Stop using the cleaner, solvent, or adhesive that produced the change. Do not apply another chemical in an attempt to reverse it. Consult material-specific compatibility information and determine whether the component remains suitable for its intended use.

For an important clear, stressed, or impact-exposed part, visible chemical damage is a reason to consider replacement rather than continuing the experiment.

If the bond fails within old paint or a coating

The epoxy may have adhered to the coating while the coating detached from the plastic. Determine whether that layer can be removed safely and whether direct bonding to the underlying polymer is supported by the adhesive instructions.

If the plastic or epoxy itself breaks

Failure away from the interface does not quantify the repair’s capacity or prove long-term durability. The break could reflect the material, geometry, impact, brittleness, or applied load. Treat it as one observation from a limited test, not as certification of the repair.

The practical answer to “does epoxy stick to plastic?” is conditional: compatible epoxy can bond several properly prepared rigid plastics, but ordinary epoxy is generally a poor choice for PE, PP, PTFE, silicone, and repeatedly flexed parts.

Before committing:

• Identify the polymer.
• Verify that the adhesive names it.
• Consider flexibility, load, environment, and consequences of failure.
• Clean with a plastic-compatible method.
• Abrade only when appropriate.
• Remove debris and dry the surface fully.
• Measure and mix exactly as directed.
• Fixture the repair without distorting it.
• Wait for the stated full cure.
• Test representative material before relying on the bond.

If compatibility remains uncertain—or failure would have serious consequences—choose a purpose-made adhesive, controlled plastic welding, mechanical repair, or replacement rather than assuming more epoxy will solve the problem.

Frequently asked questions

Will epoxy stick to PVC or ABS plastic?

Rigid PVC and ABS are among the more promising plastics for a compatible two-part epoxy. They should still be identified correctly, cleaned, dried, and abraded only as directed by the adhesive and plastic guidance.

Compatibility is not universal. Flexible or heavily plasticized PVC may behave differently from rigid PVC, while paint, coatings, and mould-release residue can interfere with either substrate. Confirm that the selected epoxy names PVC or ABS, then assess a representative test bond after full cure.

A dedicated PVC adhesive or another plastic-specific system may be preferable for some assemblies. The right choice depends on the exact polymer, fit, gap, service conditions, and product instructions.

Can sanding make epoxy stick to polyethylene or polypropylene?

Not reliably by itself. Sanding adds texture and may improve mechanical keying, but it does not necessarily correct the low surface energy that makes polyethylene and polypropylene difficult for ordinary epoxy to wet.

Use a system explicitly rated for PE or PP and follow its specified primer or activation process. Depending on the component, controlled plastic welding, mechanical attachment, or replacement may be more appropriate. Do not improvise chemical etching or flame treatment.

How long should epoxy cure before a plastic repair is used?

Wait for the full-cure time stated for the specific epoxy under the actual application conditions. Working time, initial set, and handling time are not the same as full cure.

Product schedules vary, and temperature can affect them. Until the stated full cure has elapsed, keep the repair free from loading and the service exposures prohibited by the product instructions.

What should I use when ordinary epoxy will not bond the plastic?

Choose according to the polymer and repair:

• PE or PP: a low-surface-energy adhesive explicitly rated for that polymer, including its specified primer or activation
• Suitable thermoplastic parts: controlled plastic welding
• Flexible plastic: a polymer-compatible flexible adhesive or mechanical repair
• PTFE or silicone: a specialist system or non-adhesive retention
• Poorly fitting or unsuitable joint: a mechanical solution, redesign, or replacement
• Consequential or badly degraded part: replacement or application-specific professional evaluation

Do not choose an alternative solely because its label says “plastic glue.” Check whether the technical documentation names the exact polymer and intended environment.

Will another coat of epoxy fix a layer that is peeling from plastic?

No. If the first layer is peeling from the plastic, the original plastic-to-epoxy interface is weak. Another coat will bond to the existing epoxy rather than directly to the plastic underneath.

Remove failed material where appropriate and investigate the cause: incorrect plastic identification, low surface energy, contamination, incompatible preparation, omitted primer, repeated flexing, or an unsuitable adhesive. For PE, PP, PTFE, silicone, or a repeatedly bending part, changing the repair method is generally more useful than adding epoxy.