How To Fill Gaps Between Weatherboard and Door Frame

What sealer or filler is best for a 3–6mm gap between a weatherboard and a door frame?

Selleys names three products for this joint, and the right one depends on how you want to clean up and whether you intend to paint over the finished bead. Selleys 650 FC Adhesive/Sealant and Selleys ProSeries Flexiseal are both suitable for sealing these gaps. Both remain flexible once cured, and both are mineral turps clean-up. If you would rather work with something you can wipe off with water, Selleys No More Gaps Exterior is also fine for this job.

The one detail worth checking before you buy is the gap itself. A weatherboard-to-frame joint that measures 3mm at the narrow end sits below the minimum joint size the construction sealants are specified for, and that changes which of the three products actually suits your wall.

Why this particular joint moves

The junction between a weatherboard wall and a door frame is one of the harder joints on a house to keep sealed. Timber weatherboards take up and give off moisture with the weather and move across their width as they do. The frame is fixed. The gap between them therefore opens and closes seasonally, and it does so while sitting in full sun, rain and wind.

That is why a rigid filler fails here within a season or two and why all three of the products Selleys names for this job are flexible rather than hard-setting. It is also why the products carry a published movement capability — the percentage of the joint width the cured bead can absorb without tearing or pulling off the sides.

The three options compared

Property	650 FC Adhesive/Sealant	ProSeries Flexiseal	No More Gaps Exterior
Technology	Silane modified polymer (SMP)	Polyurethane construction sealant	Water-based
Joint movement capability	±20%	±25%	±25% (ISO 9046)
Clean-up	Mineral turps	Mineral turps	Water, with a damp cloth
Skin time	20 minutes (23°C, 50% RH)	Not published for this SKU	30 ± 5 minutes
Cure rate	2.5mm / 24 hours	Not published for this SKU	Protect from water for 24 hours
Application temperature	5–35°C	Applies better around 20°C	5–40°C, not below 10°C
Colours	Grey, black, white	Range of colours	White
Paintable	Yes, once cured (water-based paint, or prime first for solvent-based)	Yes, after cure	Yes, once skinned

These are indicative figures from the products' technical data sheets, not values intended for preparing specifications.

What the numbers mean for a 3–6mm gap

Minimum joint size is the deciding factor. The 650 FC technical data sheet specifies a minimum joint width and depth of 6mm, and an optimum width-to-depth ratio of 2:1, with a 1:1 ratio for joints up to 12mm wide. A 6mm gap sits exactly at that minimum. A 3mm gap does not — there is not enough cross-section of cured sealant in a 3mm bead to stretch and recover the way the material is designed to.

In practice that means: where your gap opens out to 5–6mm, 650 FC or ProSeries Flexiseal will do the job properly. Where it narrows to 3mm, either open the joint out slightly with a scraper so the sealant has room to work, or use No More Gaps Exterior, which is formulated for gaps in exactly this range — its stated maximum is 10mm wide by 5mm deep.

Movement capability of ±20% to ±25% on a 6mm joint means the bead can tolerate roughly 1.2 to 1.5mm of opening and closing. That is generous for a weatherboard junction under normal seasonal movement, which is why all three are appropriate here.

Cure rate of 2.5mm per 24 hours for 650 FC tells you the bead is not fully cured the day you apply it. A 6mm-deep bead needs the better part of three days to cure through. It will be weatherproof on the surface long before that, but do not disturb it.

Preparing the joint

1. Rake out any old, crumbling caulk. All three products need surfaces that are clean, dry and free of dust, grease and flaking or powdery paint.
2. Check the joint is dry. Applying a polyurethane sealant to a substrate holding high moisture can cause bubbling within the bead as it cures.
3. Where the gap is deeper than it is wide, fit a polyethylene foam backing rod. This controls the depth of the bead and — more importantly — stops the sealant bonding to the back of the joint. Sealant should adhere to the two sides only, so it is free to stretch.
4. Mask both sides if you want a clean line, and pull the tape off before the sealant skins.
5. Gun the sealant in, pushing it ahead of the nozzle so it makes full contact with both faces rather than bridging across the top of the gap.
6. Tool the bead with a spatula, forcing it into the joint. Use a wet spatula for No More Gaps Exterior; a dry or soapy spatula for the two construction sealants.
7. Clean up straight away — water for No More Gaps Exterior, mineral turps for 650 FC and ProSeries Flexiseal.

Painting over the bead

All three are paintable, but the timing differs and it matters on an exterior wall.

No More Gaps Exterior needs to skin — roughly 30 minutes — before you paint with water-based paint, and 24 to 48 hours before oil-based paints. Selleys' own advice is to avoid applying paint too thickly, and to allow longer drying for flat and ceiling paints, which contain less binder and so flex less as the sealant dries underneath. On bare exterior timber, put a coat of acrylic paint down and let it dry before the sealant goes in, then paint over with a quality exterior paint.

650 FC can be painted once cured, with water-based paints. For solvent-based paints, prime with a water-based primer first. Note also that the sealant may affect the drying of alkyd or oil-based paints, and that white sealant may yellow slightly with age and sun exposure.

Limitations worth knowing before you start

• Do not apply below 5°C (650 FC and ProSeries Flexiseal) or below 10°C (No More Gaps Exterior, which also must not be stored below 10°C). A cold morning on a south-facing wall is enough to put you outside these limits.
• 650 FC requires atmospheric moisture to cure. It is not suitable for continuous submersion or below-waterline use — irrelevant for a door frame, but relevant if you are tempted to use the leftover elsewhere.
• Do not apply either construction sealant in the presence of a curing silicone or polyurethane sealant — the cure of the fresh bead can be affected. Finish one product before starting another nearby.
• Avoid solvent cleaners on the joint while the sealant is curing. Contact with solvents during cure will affect it.
• No More Gaps Exterior is not for sealing around bathtubs or kitchen sinks, and Selleys directs you to a silicone sealant where excessive joint movement occurs or where the joint is prone to frequent water immersion.
• ProSeries Flexiseal and 650 FC are classified as hazardous. Wear gloves and safety glasses, and read the safety data sheet before use.

Common questions about sealing weatherboard-to-frame gaps

Which of the three is easiest to use?

No More Gaps Exterior, because it cleans up with water and a damp cloth while wet. The other two need mineral turps, which means having a rag and solvent ready before you start.

Will the sealant stick to painted weatherboard?

It needs to stick to sound paint. Flaking or powdery paint has to come off first — the bead is only as well attached as the layer underneath it. On bare exterior timber, No More Gaps Exterior performs best over a dried coat of acrylic paint.

Can I use these on the inside face of the door frame too?

Yes. All three are suitable for indoor and outdoor use.

My gap is closer to 10mm in places — what then?

10mm is the stated maximum width for No More Gaps Exterior (at up to 5mm depth). For 650 FC, joints up to 12mm wide use a 1:1 width-to-depth ratio, and joints wider than 12mm use 2:1 with a backing rod. Fit a foam backing rod in the wider sections either way.

How long before the bead is rain-safe?

No More Gaps Exterior needs to be protected from water for 24 hours to let it strengthen. 650 FC skins in about 20 minutes and cures at 2.5mm per 24 hours; the skin is what sheds a shower, the cure is what gives it strength.

Do I need a backing rod on a 6mm gap?

Only if the gap is deeper than it is wide. The point of the rod is to keep the sealant off the back of the joint so it can stretch between the two sides. On a shallow weatherboard junction it is often unnecessary.

Can I paint the bead the same day?

With No More Gaps Exterior and a water-based paint, yes — once it has skinned, around 30 minutes. With oil-based paint, wait 24 to 48 hours. With 650 FC, wait until it has cured.

Will the white sealant stay white?

White 650 FC may yellow slightly with age and sun exposure. If the joint is in direct sun and the colour matters, paint over it or choose a colour that suits the finished wall.

What removes it if I make a mess?

While wet: water for No More Gaps Exterior, mineral turps for the other two. Once cured, 650 FC and ProSeries Flexiseal can only be removed mechanically; cured No More Gaps Exterior has to be cut, and the remainder can be softened with acetone.

Is there a silicone option instead?

For a joint that moves a great deal or that sees frequent water, Selleys directs you to an appropriate silicone sealant rather than a water-based gap filler. See types of sealants for how the families differ.