How To Fix a Broken Tail Light

What adhesive is best to seal and glue a broken car tail light reflector?

Selleys recommends a silicone for this repair — specifically Selleys Roof & Gutter Silicone. It should form a strong bond and a waterproof seal on the surface of the tail light, which is what this repair actually needs: the housing has to keep water out of the lamp, and it has to stay flexible while the car twists, vibrates and heats up.

Roof & Gutter is a neutral-cure, 100% silicone sealant with excellent UV and weather resistance. Neutral cure is the important part on a vehicle — it does not release acid as it cures, so it is non-corrosive around the metal and electrical hardware inside a lamp assembly.

Before you start: check the plastic

This is the step that decides whether the repair works, and it is worth doing properly.

The Roof & Gutter Silicone data sheet lists the substrates it is made for. On the non-porous side that is aluminium, steel and stainless steel, mild and galvanised steel, powder-coated steel, and PVC. It states plainly that it is not for use on polyethylene, polypropylene or PTFE, and it instructs you to pre-test compatibility with substrates before use.

Automotive lens and housing plastics are not on that list. Lenses are commonly acrylic or polycarbonate and housings are frequently ABS or a polypropylene blend, and the recycling mark moulded into the back of the housing will usually tell you which. If it says PP or PE, this is not the product. If it is something else, test on a hidden part of the housing first — bond a small amount, let it cure, and try to peel it off. Selleys recommends users conduct their own risk assessment and apply a sample to the intended area to confirm acceptable appearance, finish and performance before committing.

The technical figures

Property	Typical result
Technology	100% silicone, neutral / moisture curing
Skin time	5–10 minutes
Tooling time	5 minutes
Tack-free time	30 minutes
Curing rate	2 mm / 24 hours
Joint movement	±25%
Elongation at break	330% (ASTM D412)
Shore A hardness	32
Application temperature	+5°C to +30°C
Service temperature	−60°C to +150°C
Shelf life	18 months

These figures are based on current product material data, quality test methods and testing conditions such as temperature, moisture and substrate.

What the numbers mean for a tail light

The service temperature range is the reassuring one. At −60°C to +150°C, a rear lamp cluster is comfortably inside it, including the heat from an incandescent bulb in a confined housing.

The elongation and movement figures are why silicone suits this and a rigid glue does not. At 330% elongation and ±25% joint movement, the cured bead flexes with the body panel rather than cracking along the repair line the first time you go over a speed hump. A rigid epoxy repair to a plastic housing tends to fail at the edge of the bond for exactly this reason.

The cure rate sets the timetable. At 2 mm per 24 hours, a thin sealing bead is functional within a day, but a thicker fillet in a corner of the housing takes proportionally longer. Do not put the car back in the rain on the strength of a surface that has gone tack-free at 30 minutes — tack-free is not cured.

Colour matters more than you would think. Roof & Gutter comes in a range of colours including black, grey and translucent. Translucent is the sensible choice on a lens, black on a dark housing. It is not paintable, so pick the colour you want to live with.

How to do the repair

1. Remove the lamp assembly from the car if you can. A repair done on the bench, with the housing dry and accessible from both sides, is far better than one done in situ.
2. Identify the plastic and pre-test the silicone on a hidden area. Let the test cure before you rely on it.
3. Ensure all surfaces are clean, dry, sound and free of dust, oil and other contaminants. Clean the non-porous surfaces with a clean cloth dampened with isopropyl alcohol or white spirit, wiping in one direction only, then immediately wipe with a clean, dry cloth.
4. Remove all existing adhesive or sealant from any previous repair.
5. Cut the cartridge tip, screw on the nozzle and trim it to the bead size you want. Use a good-quality gun.
6. Apply a continuous bead along the crack or joint, working the silicone into it rather than laying it on top. For a missing fragment, a patch of matching plastic bedded in silicone gives a much neater and stronger result than silicone alone.
7. Tool the bead with a clean spatula dipped in soapy water before a skin forms — you have around 5 minutes.
8. Support the parts and leave the assembly undisturbed while it cures. Refit only once it has cured properly.
9. Clean excess off tools with turps. Cured silicone has to be removed mechanically or with Selleys Sealant Remover.

Limitations

• Not for use on polyethylene, polypropylene or PTFE. Check the moulding mark on the housing.
• Pre-test compatibility with substrates before use. Automotive plastics are not among the listed substrates.
• Not paintable. Allow paint to dry for at least 7 days before applying the sealant on top of it.
• Not recommended for continuous submersion or below-waterline use. Shedding rain is fine; a permanently flooded housing is not — and if the lamp is filling with water, find and fix the drain rather than sealing it shut.
• Do not apply below +5°C or above +40°C ambient or substrate temperature.
• Avoid contact with materials containing high levels of plasticisers, such as rubber and bituminous substrates. Old lamp gaskets are often exactly this.
• The sealant requires atmospheric moisture to cure. On non-porous substrates or in low humidity, curing slows — and a lamp housing is entirely non-porous.

Common questions about repairing a tail light

Will silicone hold a cracked lens together?

Selleys says a silicone such as Roof & Gutter should form a strong bond and waterproof seal on the surface of the tail light. It is best used as a seal supported by a mechanical repair — a backing patch — rather than as the sole structure across a large break.

Which colour should I use?

Translucent on a lens, or a dark colour on a housing. Roof & Gutter comes in a wide range of colours and is not paintable, so choose before you apply.

How long before I can drive in the rain?

Cure runs at 2 mm per 24 hours. Tack-free at 30 minutes is not cured. Allow a full day for a thin bead and longer for anything thicker.

Will engine or bulb heat affect it?

Its service temperature is −60°C to +150°C, which covers a rear lamp cluster.

Is it safe around the wiring and metal contacts?

It is a neutral cure silicone, so it does not release acetic acid during cure the way an acetic cure product does.

What if the housing is polypropylene?

Do not use it. The data sheet excludes polyethylene, polypropylene and PTFE. For difficult plastics Selleys directs you to All Plastics.

Should I use an epoxy instead?

Selleys nominates a silicone here. An epoxy gives a rigid joint that does not flex with the body, and the Araldite range will not bond polyethylene or polypropylene either.

Can I paint over the repair?

No. Roof & Gutter Silicone is not paintable.

How do I clean it up?

Turps for excess on tools and equipment, before it cures. Cured sealant needs mechanical removal or Selleys Sealant Remover.

Is there a stronger option for a structural crack?

For a housing that has broken into pieces, a plastic-specific system is a better fit than a sealant. Check the plastic type first, since that decides what is available to you.