Best Sealant To Seal Plexiglass to Aluminium
What sealant seals a plexiglass window to an aluminium caravan frame?
Selleys recommends Selleys Roof & Gutter Silicone to seal a plexiglass window to the aluminium frame of a caravan. It gives a superior waterproof seal that is resistant to lifting and provides a long-lasting flexible joint, and it comes in a wide range of colours.
One critical caveat before you buy it. Selleys Roof & Gutter may not dry or cure properly if a polyurethane sealant has been used in that joint previously. If that is the case — and on a caravan it very often is — use a polyurethane instead, such as Selleys ProSeries Flexiseal.
Why the polyurethane question comes first
This is not a minor footnote. Roof & Gutter Silicone is a neutral (oxime) cure silicone, and neutral cure chemistry is sensitive to the presence of curing polyurethane. Where the two meet, the cure can be inhibited: the bead stays soft, does not develop its properties, and the seal fails. The same incompatibility runs the other way — polyurethane sealants should not be applied or cured in the presence of curing silicone.
Caravans are resealed repeatedly over their lives, often with whatever product was to hand, and polyurethane is the industry standard for van joints. So before you choose:
- Strip the old sealant out completely rather than beading over it.
- If you know or suspect a polyurethane was used there, go with the polyurethane.
- If in doubt, apply a small test bead of the silicone in a hidden part of the cleaned joint and check the next day that it has cured properly.
Why Roof & Gutter Silicone suits the aluminium side
The joint has aluminium on one side, and Roof & Gutter Silicone is specifically intended for metal substrates — aluminium, sheet metal and galvanised iron among them. Being neutral cure, it does not release acetic acid as it cures, so it does not carry the "not suitable for metals other than aluminium and stainless steel" restriction that acid-cure silicones do.
| Technology | Neutral cure silicone, 100% silicone |
| Specific gravity | Approximately 1.04 |
| Skin time | 5 to 10 minutes |
| Tooling time | 5 minutes |
| Tack-free time | 30 minutes |
| Cure rate | 2mm per day |
| Slump at 25°C | 0mm |
| Shore A hardness | 32 |
| Elongation at break | 330% |
Indicative data only, not intended for the preparation of specifications.
The 5 minute tooling time is the tightest figure on this page. It is far shorter than most sealants, and it dictates how you work: seal a short length, tool it immediately, move on. Trying to run a whole window perimeter before tooling will leave you with a skinned bead you cannot finish.
Colours
The 300g cartridge range is largely Colorbond colour matched — Classic Cream, Cottage Green, Deep Ocean, Dune, Ironstone, Manor Red, Monument, Paperbark, Surfmist, plus Galvanised Grey, White and Woodland Grey. Black, Grey and Translucent are available in 310g cartridges, and Translucent also in a 75g tube. On a window frame, translucent or the closest match to the van body is usually the least conspicuous.
Test the plexiglass
The published substrate list for Roof & Gutter Silicone covers metal and tile roofing, Colorbond, guttering, flashings, skylights, air conditioning units, aluminium, sheet metal and galvanised iron. Acrylic sheet is not named individually.
Bond a test bead to an offcut of the same sheet, let it cure, and check adhesion before you seal the window itself. The surface preparation Selleys specifies — cleaning with mineral turpentine and wiping dry with a clean rag — is directed at metal and glass surfaces, so use it on the aluminium frame and test it on an offcut of the sheet before applying it to the window.
How to seal the window
- Remove the old sealant completely, cutting and scraping it out of the joint.
- Clean all surfaces so they are free of dirt, dust, oil and loose rust. Clean the metal with mineral turpentine and wipe dry with a clean rag.
- Let everything dry thoroughly.
- Cut the nozzle to the joint width, at a 45° angle, and load a caulking gun.
- Apply at 45°, pushing the sealant into the joint so it contacts both the frame and the sheet.
- Tool within 5 to 10 minutes of application. Work in short lengths.
- Clean up uncured product with mineral turpentine before the sealant skins.
- Allow the cure to run at 2mm per day; tack-free is at 30 minutes but that is not a cure.
Limitations
- Not paintable. Complete all painting at least seven days before applying the sealant.
- May not cure properly where a polyurethane sealant has been used.
- Not recommended for plugging burst pipes or below waterline applications.
- Not suitable for aquarium construction.
- Not suitable for permanent water immersion, or submerged joints where porous substrates let water reach the bond interface.
Common questions
What if polyurethane was used on this joint before?
Use a polyurethane instead — Selleys ProSeries Flexiseal. The silicone may not dry or cure properly over polyurethane residue.
Will it stick to plexiglass?
Acrylic is not individually named on the data sheet. Test a bead on an offcut and check adhesion after it has cured.
Will it corrode the aluminium frame?
It is a neutral cure silicone, so it does not release acetic acid during cure. It is intended for aluminium, sheet metal and galvanised iron.
How long do I have to tool the bead?
Five minutes of tooling time, with skin forming at 5 to 10 minutes. Work in short sections.
How long does it take to cure?
2mm per day. Tack-free at 30 minutes is not the same as cured.
Can I paint over it?
No. Painting must be completed at least seven days before the sealant is applied.
What colours are available?
A large Colorbond-matched range in 300g cartridges, plus Black, Grey and Translucent in 310g, and Translucent in a 75g tube.
Is there a guarantee?
It carries a 25 year guarantee against cracking and peeling, when used as directed for private use and within 12 months of purchase.
Can I use it below the water line on a boat?
No — it is not recommended for below waterline applications or permanent water immersion.
Is there a related answer for caravans?
Yes — the best sealant for caravan joints and resealing a seam in a caravan.
What is the best sealant to seal plexiglass to an aluminium caravan window frame?
Selleys MarineFlex is recommended as the best sealant for sealing plexiglass to aluminium, such as fixing a minor leak in an aluminium caravan window frame.
Why is Selleys MarineFlex suitable for both aluminium and plexiglass?
Selleys MarineFlex is a flexible above and below waterline sealant that can be applied directly to aluminium without needing a primer, and it can also bond effectively to plexiglass (polycarbonate) without primer, provided the surface is properly prepared.
Do I need to prepare the plexiglass surface before applying MarineFlex?
Yes, light sanding of the plexiglass surface is recommended before applying Selleys MarineFlex to improve surface adhesion.
Is priming required before applying MarineFlex to aluminium or plexiglass?
No, Selleys MarineFlex does not require a primer for either aluminium or polycarbonate (plexiglass) substrates.
Can Selleys All Clear Multi-Purpose Sealant be used on plexiglass?
No, Selleys All Clear Multi-Purpose Sealant should not be used on plexiglass, as it may cause damage to the material.
Is standard PRO-TRADE Glass Sealant suitable for plexiglass?
No, standard PRO-TRADE Glass Sealant is not suitable for plexiglass applications.
What is an alternative sealant if Selleys MarineFlex is not available?
Selleys Liquid Nails Flex is a compatible alternative, as it can bond to both aluminium and polycarbonate as non-porous substrates, though surface preparation remains important.