Silicone Compatible With Laserlite Polycarbonate

What silicone is compatible with Laserlite polycarbonate sheeting?

The honest answer is that no sealant is guaranteed compatible with polycarbonate roofing, and sealing it at all carries a real risk. Selleys' advice on this question is deliberately cautious: you have to be careful when sticking or sealing polycarbonate sheeting, because many of the main sheet manufacturers will void their warranty if any type of product is applied to their sheets. Sealants and adhesives can also cause a condition called crazing — fine hairline cracks that appear across the surface of the polycarbonate and cannot be undone.

If you have read that, checked your sheet manufacturer's own warranty terms, and are willing to accept the risk, the Selleys product nominated for the job is Selleys Roof & Gutter Silicone. Before you commit to a full run, apply a sample to an offcut or an inconspicuous area and leave it — that is what Selleys recommends for this product on any substrate it does not explicitly list, and polycarbonate is one of those.

Why crazing matters more than adhesion here

Most sealing questions are about whether the bead will stick. On polycarbonate the more important question is what the sealant does to the sheet itself. Crazing is a network of fine surface cracks in the plastic. Once it starts, it does not stop at the bead — it spreads through the stressed area around it, and the affected sheet is cosmetically ruined and structurally weaker at exactly the point you were trying to seal. That is why the manufacturer warranty exclusion exists, and why Selleys frames this as a risk decision rather than a product recommendation.

The practical consequence: if the sheet is still under warranty, or is in a visible position where a hazed patch would bother you, a mechanical fix specified by the sheet manufacturer is the lower-risk answer.

What Roof & Gutter Silicone actually is

Roof & Gutter Silicone is a neutral-cure, 100% silicone sealant. Neutral cure matters here: it does not release acetic acid as it sets, which is one less aggressive chemistry acting on a plastic sheet. It is UV and weather resistant, which is what suits it to a roof, and it carries a 25-year guarantee against crumbling, cracking or peeling away when used as directed for private use within 12 months of purchase.

Property	Typical result
Technology	Silicone, moisture curing (neutral cure)
Tooling time	5 minutes
Skin time	5–10 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 indicative product data based on Selleys' quality test methods and testing conditions, not values for preparing a specification.

What the numbers mean for blocking old screw holes

A screw hole in roof sheeting is a very small joint in a very exposed position, and two of the figures above decide how you approach it.

The 2 mm per 24 hours cure rate tells you the bead cures from the outside in. If you push silicone into a hole that is, say, 6 mm deep, the outer skin forms within 5–10 minutes but the material at the bottom is still uncured a day later. That is fine structurally, but it means you must not disturb, walk on or wash the area for well over a day, and it means overfilling a deep hole does not make it seal faster — it makes it cure slower.

Skin time of 5–10 minutes is your entire tooling window. Roof work usually means working in sun and wind, both of which shorten it. Do one hole, tool it immediately with a clean spatula dipped in soapy water, then move on. Do not run a dozen holes and come back to smooth them.

Selleys also notes that this sealant needs atmospheric moisture to cure. On a non-porous substrate — and polycarbonate is about as non-porous as it gets — or in dry conditions, curing slows down. Plan for longer than the table suggests if you are working in a dry inland summer.

Preparation and application

1. Check your sheet manufacturer's warranty terms first. If applying a sealant voids the warranty and the roof is still covered, stop here.
2. Apply a small test patch to an offcut of the same sheet, in the same conditions, and leave it. Look for hazing or fine cracking before you commit.
3. Make sure the surface is clean, dry, sound and free of dust, oil, rust and other contaminants.
4. For non-porous surfaces, wipe with a clean cloth dampened with isopropyl alcohol or white spirit, in one direction only, then wipe immediately with a clean dry cloth. Test any solvent on an offcut first — solvents are themselves a crazing risk on polycarbonate.
5. Remove all existing adhesive or sealant from the area before resealing.
6. Cut the cartridge tip, screw on the nozzle and trim it to the bead size you want. A high-thrust caulking gun makes a high-viscosity silicone far easier to control.
7. Apply a continuous bead, pushing the sealant into the hole rather than laying it over the top.
8. Tool the sealant with a clean spatula dipped in soapy water before a skin forms.
9. Clean tools and excess with turps. Once cured, silicone has to come off mechanically.

Limitations you must not ignore

• Polycarbonate is not on the approved substrate list. Roof & Gutter Silicone's published substrates are timber and plywood, fibre reinforced cement, bricks, concrete and masonry on the porous side, and aluminium, steel and stainless steel, mild and galvanised steel, powder-coated steel and PVC on the non-porous side. Selleys explicitly requires you to pre-test compatibility with substrates before use.
• Not for use on polyethylene, polypropylene or PTFE (Teflon).
• Do not apply if the ambient or substrate surface temperature is below +5 °C or above +40 °C. A dark roof in midsummer will exceed that easily by mid-morning, so work early.
• Not recommended for continuous submersion or below-waterline use.
• Not for structural glazing. Sealing a hole is not the same as relying on the sealant to hold a sheet in place.
• Avoid contact with materials containing high levels of plasticisers, such as rubber and bituminous substrates.
• Not paintable. If the area is to be painted, allow the paint to dry for at least 7 days before applying the sealant over it.

Removing it later — an important catch

If the repair does not work out, cured silicone normally comes off with Selleys Sealant Remover. That product is not suitable for polycarbonate, along with rubber and EPDM. So on a Laserlite sheet your only removal option is careful mechanical cutting and scraping — which is another reason to test on an offcut before you seal the real thing.

Safety: solvents on a roof, and why the remover is doubly excluded here

Roof & Gutter Silicone is a Warning-level product — eye irritation and skin sensitisation. The classified product on this page is the removal option, and the reason it matters is that everything here happens at height.

Selleys Sealant Remover is classified Danger, Poison Schedule S5, with H304 (may be fatal if swallowed and enters airways), H226 (flammable liquid and vapour), H336 (may cause drowsiness or dizziness) and H315 (causes skin irritation). Dangerous Goods Class 3, UN 1133, Packing Group II.

It is excluded twice over on Laserlite. The section above already gives the first reason: its data sheet rules out polycarbonate, so it cannot touch the sheet. The second is H336. Narcotic-effects Category 3 means headache, dizziness and drowsiness from a single exposure to accumulated vapour, and the one place that is genuinely dangerous is standing on a pitched roof. If you use it at all on this job it belongs on the metal flashings and gutters at the end of the page, not on the sheeting, and not while you are balanced on a batten.

The same logic applies to the cleaning solvents the method calls for. Step 4 has you wiping the surface with isopropyl alcohol or white spirit. Those are flammable and they evaporate; a dark roof in midsummer is already past 40 °C, which is what drives them off faster. Work early, keep the container capped and off the hot sheet, and keep it away from a flue, a solar collector or a rooftop air conditioner.

The lower-hazard route is the one this page recommends anyway: if the repair has to come off a polycarbonate sheet, careful mechanical cutting and scraping is the only option, and it carries no classification. Test on an offcut first, as the page says, and you are far less likely to need to remove anything at all.

Gloves, eye protection, three points of contact on the ladder, and do not work alone at height. Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions

Which Selleys silicone is recommended for polycarbonate?

Selleys Roof & Gutter Silicone, but only if you accept the risk. Selleys' position is that sealants and adhesives may cause crazing on polycarbonate and that many sheet manufacturers void their warranty if any product is applied.

What is crazing?

A condition where fine hairline cracks appear on the surface of the polycarbonate. It is caused by the product applied to the sheet and it cannot be reversed.

Will Roof & Gutter Silicone stick to polycarbonate?

Polycarbonate is not one of the substrates listed on the product's technical data sheet, so there is no published adhesion figure for it. Selleys requires a pre-test on the intended surface to confirm appearance, finish and performance.

How long before the repair is weatherproof?

It skins in 5–10 minutes and is tack-free in about 30 minutes, but it cures at 2 mm per 24 hours, so a bead of any depth takes more than a day to cure right through. Do not disturb it in the meantime.

Can I do this on a hot day?

No. Do not apply if the ambient or substrate temperature is below +5 °C or above +40 °C. Roof sheeting in direct sun passes +40 °C well before the air does.

What colours does it come in?

A Colorbond colour-matched range, plus translucent. Most are 300 g or 310 g cartridges, with a 90 ml translucent pack.

Can I paint over the repair?

No, it is not paintable. If the surrounding area needs paint, paint first and allow at least 7 days before sealing.

How do I remove it if I get it wrong?

Mechanically. Selleys Sealant Remover is not suitable for polycarbonate, so cutting and scraping is the only option on this substrate.

What if the hole is in a gutter or flashing rather than the sheet?

That is what Roof & Gutter Silicone is designed for. It is listed as ideal for sealing joints around the roof, gutters, flashings, skylights and tile roofing, and for common roofing substrates such as Colorbond sheet and coated metal panels — no crazing concern applies to metal.