How To Fill Gaps in Aluminium and Timber Window Frames

What should you use to fill the gap between an aluminium window frame and the adjoining timber frame?

Use Selleys No More Gaps Exterior. It is the product Selleys specifies for the joint between aluminium window frames and the timber frames they sit in, and the same product covers gaps between timber weatherboards, fences, eaves and doorframes. Its technical data sheet lists both timber and aluminium among the substrates it is suitable for, which is exactly why it works on a joint that has one of each on either side.

The thing worth understanding before you start is why this particular joint fails. An aluminium frame and a timber frame move differently — aluminium expands and contracts with temperature, timber swells and shrinks with humidity — so the gap between them is never a static width. That is a movement joint, and it needs a filler that can stretch and compress rather than one that sets hard. No More Gaps Exterior is rated at ±25% movement capability (ISO 9046), with elongation at break of 500% (ASTM D412). That is where the long-term resistance to cracking and crumbling comes from.

The published figures, and what they mean for this job

Technology	Water-based
Colour	White
Packaging	430g cartridge
Application temperature	5 to 40°C
Skin time	30 ± 5 minutes
Tack-free time	45 ± 5 minutes
Movement capability	±25% (ISO 9046)
Elongation at break	500% (ASTM D412)
Shore A hardness	> 40
Tensile strength	0.6 ± 0.05 MPa
Maximum gap	10mm wide × 5mm deep

Selleys notes that these are indicative figures only, not intended for the preparation of specifications.

Two of those numbers matter more than the rest on a window reveal. The 10mm wide by 5mm deep maximum is the one people ignore, and it is the most common cause of paint cracking over a filled window joint. A deeper bead holds more water, shrinks more as that water leaves, and drags the paint film with it. If the gap between your frames is deeper than 5mm, do not simply fill it to the back — build it up in layers, letting each one dry.

The 30 minute skin time is the second. That is your window to tool the bead before the surface sets, and it is also the point at which you can start painting.

How to fill the joint

1. Make sure both surfaces are clean, dry and free from dust, grease, flaking or powdery paint, and any old or crumbling caulk. Aluminium in particular tends to carry a film of grime that a filler will happily stick to instead of sticking to the frame.
2. Cut the nozzle at a 45° angle, cut the cartridge tip and fit the nozzle. Insert the cartridge into a caulking gun.
3. Apply with the caulking gun, pushing the product into the gap or joint rather than laying it across the surface.
4. Smooth out with a wet spatula, making sure enough product stays in the joint. It is easy to tool so hard that you pull most of the bead back out.
5. Clean up wet product with a damp cloth immediately.
6. Allow the surface to skin before painting — approximately 30 minutes.
7. Protect the joint from water for 24 hours so the product can strengthen.

Painting over it

No More Gaps Exterior is paintable, which is the main reason to choose it over a silicone on an exposed window frame. The guidance for avoiding paint cracking is specific:

• Keep the gap no larger than 5mm deep and 10mm wide, and apply at temperatures above 10°C.
• Let the sealant dry before painting over it, and avoid applying the paint too thickly.
• Allow 24 to 48 hours before painting with oil-based paints.
• Flat and ceiling paints need longer drying times, because they are highly filled and contain less binder — the paint does not flex as much as the sealant dries.
• On bare exterior surfaces, apply a coat of acrylic paint and let it dry before applying the filler, for maximum durability. Then finish with a quality exterior paint.

Limitations — where No More Gaps Exterior should not be used

• Maximum gap 10mm wide by 5mm deep.
• Not for sealing around bathtubs, kitchen sinks and similar. Use No More Gaps Bathroom & Kitchen there instead.
• Where excessive joint movement occurs, or in areas prone to frequent water immersion, use the appropriate Selleys silicone sealant rather than this product.
• Not to be stored or applied below 10°C.

Selleys also notes that in harsh conditions, joints between boards can expand beyond the movement capability of the product. If that happens, re-apply and paint over again. On a new weatherboard installation, check the board manufacturer instructions before using the product.

Safety: gloves for the tooling, and what changes if the reveal is too deep

No More Gaps Exterior carries the signal word Warning and one hazard statement: skin sensitisation Category 1, H317, may cause an allergic skin reaction. Its safety data sheet records it as not irritating to eyes or skin, not a respiratory sensitiser, not an aspiration hazard, and not classified as a carcinogen or reproductive toxicant. For a product used outdoors on a window reveal, that is a short list.

H317 is not irritation, and the difference decides how you tool the bead. Irritation settles once contact stops. Sensitisation is the immune system learning to react to the material: generally permanent once acquired, triggered thereafter by far smaller exposures, and worsening with each contact. It is built by repeated casual contact — which is precisely what step 4 above becomes if you smooth the bead with a bare wet finger, window after window. Use the wet spatula, or a gloved finger. Once cured the filler is inert; the hazard belongs to the wet material.

The escalation on this page is the part that changes category entirely. Where the reveal is deeper than 5mm, this page points to No More Big Gaps. That is an isocyanate polyurethane foam classified Danger, carrying H222 (extremely flammable aerosol), H334 (may cause allergy or asthma symptoms or breathing difficulties if inhaled), H351 (suspected of causing cancer), H372 (damage to organs through prolonged or repeated exposure) and H362 (may cause harm to breast-fed children).

A window reveal is the textbook case for taking H334 seriously. You are dispensing into a cavity, often from inside with the window shut against the weather, and the vapour has nowhere to go. Respiratory sensitisation is permanent once acquired and there is no safe re-exposure afterwards. If you use it, use an organic vapour respirator rather than a dust mask, ventilate actively, wear gloves and eye protection, and keep the room clear of other people until the foam has cured and the space has been aired. Anyone breastfeeding should not use it at all.

The better route is the one this page already gives: do not fill a deep reveal to the back. Build it up in layers with the exterior filler, keeping each pass within 10mm wide by 5mm deep. That keeps you on a product whose entire classification is one skin statement.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions about filling window frame gaps

Will No More Gaps Exterior stick to aluminium?

Yes — aluminium is one of the substrates listed on its technical data sheet, along with timber weatherboard, synthetic weatherboard, FC sheet, brick and masonry. The surface still has to be clean and dry.

Can I paint over it?

Yes, once it has skinned, which takes about 30 minutes. Allow 24 to 48 hours before using an oil-based paint, and longer than 30 minutes for flat or ceiling paints.

What colour is it, and what size?

White, in a 430g cartridge. It is designed to be painted, so the base colour matters less than it would with a silicone.

How wide a gap can I fill?

Up to 10mm wide and 5mm deep. Anything larger should be built up in layers or filled with a product intended for bigger voids, such as No More Big Gaps.

Can I use it inside as well?

It is formulated for exterior work. For interior skirtings, cornices and door frames, No More Gaps Interior Multipurpose is the matching product.

How soon can the joint get wet?

Protect it from water for 24 hours after application so the product can develop strength.

What if the window catches driving rain?

It is UV resistant and formulated with a titanium compound to protect against UV and weather damage, so ordinary exposure is fine. If the joint is genuinely subject to frequent water immersion, the data sheet directs you to a silicone sealant instead.

How do I clean up excess?

Wet product comes off with water and a damp cloth, straight away. Cured product has to be cut, and what remains can be softened with acetone and then removed — keep acetone away from paint and plastics.

Can I use it on a fence or on eaves?

Yes. Selleys nominates timber weatherboards, fences, eaves and doorframes as suitable uses for this product.

Is there a step-by-step guide for this job?

Yes — how to fill gaps around windows walks through it in more detail.