How to: seal joints with Sikaflex 11 FC – step by step
Contents
What you need
Project template “Expansion and connection joints on the façade”The short answer: Sikaflex 11 FC is a 1-component PU adhesive and sealant for joints with low to medium movement – permissible total movement 15 %. The joint is cleaned, filled behind with a PE round cord (width 10–35 mm, for floor joints width to thickness 1:1 to 1:0.8), primed on absorbent substrates, applied bubble-free at +5 °C to +35 °C and smoothed before skin formation with smoothing agent and smoothing spatula. The sealant cures through air humidity – afterwards it is elastic and sandable.
These instructions take you through a complete joint sealing with Sikaflex 11 FC Purform – from dimensioning the joint to checking the finished joint. Typical applications are floor-to-wall connection joints, connection joints at doors, seam and lap joints in timber and metal construction, roofing work as well as joints on terraces and flat roofs. All values in these instructions come from the manufacturer's information on the products used.
Shopping list
Basic set (always required)
Optional – depending on the installation situation
- Soudal Primer 100 – for porous, absorbent or moisture-loaded joint flanks (concrete, plaster, masonry, natural stone). Waiting time before sealing: at least 60, at most 240 minutes.
- Otto smoothing agent normal or smoothing agent spray – for a smooth, gloss-retaining joint surface. Expressly suitable for polyurethane sealants too. Not for marble and other natural stones.
- PE round cord 5 m – the small pack size for individual rooms or repairs.
- Otto Cleaner F – for degreasing metallic substrates (bare or powder-coated) and for removing uncured PU residue. No flash-off required.
Tools
- Utility knife for cutting the nozzle (45° cut, width = joint width)
- Joint scraper or chisel for removing old joint compounds
- Vacuum cleaner and lint-free cloths
- Masking tape for clean joint edges
- White spirit for cleaning tools and soiling before curing
- Folding rule for checking joint width and joint depth
Why fill behind – and why the joint dimensions decide
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An elastic sealant may adhere to only two joint flanks. If it additionally sticks to the back of the joint, so-called 3-flank adhesion arises: the joint can no longer stretch freely, the stresses tear the material or the adhesion. The closed-cell PE round cord separates the sealant from the back of the joint and at the same time limits the joint depth. It is made closed-cell to DIN 18540, so absorbs no water, and meets fire behaviour B2 to DIN 4102.
Sikaflex 11 FC takes up a permissible total movement of 15 %. The joint width should generally lie between 10 and 35 mm; for floor joints a width-to-thickness ratio of 1:1 to 1:0.8 is to be observed. Joints that are too narrow or too deep are the most frequent cause of later cracks – more on this in the guide Silicone joint keeps cracking.
Step by step
Check the joint and remove old compound
Measure the width and depth of the joint. If the width is under 10 mm or over 35 mm, the joint has to be adapted structurally – the sealant cannot compensate for a wrongly dimensioned joint. Remove old sealant residue completely and mechanically; instructions for this are under Removing old silicone without leaving residue.
Check: No remains of old sealants may be visible in the joint any more – no PU sealant adheres to them.
Prepare the joint flanks
The joint flanks must be sound, firm, clean and dry – free of oil, grease, loose parts, cement slurry, paints, water-repellent treatments and anti-graffiti coatings. Sweep out and vacuum mineral flanks; degrease metallic substrates with Otto Cleaner F on a fresh, clean cloth – with powder-coated profiles always wipe lengthways, not in circles.
Check: Wipe test with a white cloth – no film of dust or grease may remain.
Insert the PE round cord
Choose a cord diameter that is about 20–30 % larger than the joint width, so that the cord wedges and does not wander. Ten diameters from 6 to 50 mm are available. With a blunt object – for example the joint smoother – press it evenly to the desired depth, without perforating the surface. If the closed cell skin is damaged, air can escape and cause bubbles in the joint.
Check: The cord lies at the same depth over its entire length and is undamaged.
Prime (only on porous substrates)
On absorbent joint flanks such as concrete, plaster, masonry or natural stone, and with increased moisture load, apply Soudal Primer 100 over the whole surface with a brush, on strongly absorbent substrate a second time. Application temperature +5 °C to +25 °C. The primer is solvent-based (flash point 24 °C) – ventilate well and keep away from ignition sources.
Check: Waiting time before sealing at least 60 minutes, at most 240 minutes. If the time window is exceeded, it has to be primed again.
Apply Sikaflex 11 FC bubble-free
Cut the nozzle at a slant to the joint width, insert the cartridge into the cartridge gun and apply the joint in one pass – with light advance, so that the material piles up in front of the nozzle and wets the flanks reliably. Application temperature: +5 °C to +35 °C. Application with a pneumatic or cordless gun is also possible.
Check: The joint is filled continuously, without air pockets and without interruptions. Stopping and restarting produces flaws.
Smooth before skin formation
Smoothing is done with smoothing agent and smoothing spatula – and before skin formation begins. Spray Otto smoothing agent normal generously onto joint and spatula, pull off in one stroke, remove excess material immediately. The smoothing agent can be diluted with water in the ratio 2 parts smoothing agent to 1 part water, retains the gloss of the surface and does not wash out colour pigments. Application temperature +5 °C to +35 °C. Do not use on marble and other natural stones – there is the natural stone smoothing agent for that. Technical details of the pulling technique in the guide Smoothing a silicone joint perfectly.
Check: The surface is evenly concave and without steps. If the joint draws threads or becomes dull, skin formation has already occurred – then do not rework it.
Clean and let cure
Pull off the masking tape immediately after smoothing. Clean soiling and tools before curing with white spirit – afterwards the material can be removed only with silicone remover or mechanically. Sikaflex 11 FC cures through reaction with air humidity; so do not cover or load the joint during curing.
Check: After complete curing the joint is elastic and sandable. Light finger pressure must not leave a permanent dent.
Times and limit values at a glance
| Work step | Value according to manufacturer's information | Check |
|---|---|---|
| Application temperature Sikaflex 11 FC | +5 °C to +35 °C | Measure substrate and air temperature, do not merely estimate the outside temperature |
| Joint width | generally 10–35 mm | Folding rule; floor joints additionally width:thickness = 1:1 to 1:0.8 |
| Permissible total movement | 15 % | With greater movement choose a different product |
| Primer 100: flash-off before sealing | min. 60, max. 240 minutes | Note the time; after 240 minutes prime again |
| Primer 100: application temperature | +5 °C to +25 °C | Thermometer at the substrate |
| Smoothing | before skin formation begins | Spatula pulls off cleanly, no thread drawing |
| Cleaning fresh residue | before curing with white spirit | Afterwards only silicone remover or mechanically |
| Curing through | Moisture-curing – take the concrete time from the technical data sheet or the cartridge print | Joint elastic and sandable, no material imprint with finger pressure |
| Storage of the cartridge | 12 months from the date of manufacture at +10 °C to +25 °C, cool and dry | Check the date of manufacture on the pack |
Common mistakes – and how to avoid them
- Worked without backing. The sealant adheres to the back of the joint, the joint cracks at the first movement. Always insert a PE round cord.
- Round cord perforated. A pointed tool destroys the closed cell structure, escaping air produces bubbles. Press in bluntly.
- Joint too narrow. Below 10 mm the permissible movement of 15 % cannot be taken up in calculation.
- Primer forgotten or time window missed. On absorbent concrete or plaster that reduces adhesion markedly. After 240 minutes prime again.
- Smoothed too late. After skin formation the spatula tears the surface open. Always work in sections: apply, smooth immediately, carry on.
- Smoothing agent on natural stone. Otto smoothing agent normal is not suitable for marble and other natural stones – see Stains on natural stone from the wrong silicone.
- Applied in frost. Below +5 °C application and cross-linking are not assured.
Frequently asked questions
Can I paint over Sikaflex 11 FC?
Sikaflex 11 FC is sandable and is often used in the area of connections that are painted over. Whether a specific coating is compatible must always be checked on a test area – elastic sealants move, rigid coatings crack. If a permanently paintable solution is wanted, the comparison in the guide Silicone or acrylic – which for what? is worthwhile.
Which colour should I choose?
Sikaflex 11 FC is available in plain white, concrete grey and black. Concrete grey is the standard for concrete and screed connections, black for metal and roof connections, plain white for light interior fit-out.
Do I always need a primer?
No. Sikaflex 11 FC has excellent adhesion to the usual building materials. An adhesion primer such as Soudal Primer 100 makes sense above all on porous, absorbent substrates and with increased moisture load on the joint flanks.
How many cartridges do I need?
Usage results from joint width times joint depth times length. You will find the calculation methods and a table under Working out silicone usage correctly and in the usage table.
Sikaflex 11 FC or a hybrid sealant?
Both are elastic adhesives and sealants with different strengths. You will find the direct comparison in the guide Sikaflex 11 FC or Fix All Flexi. Further products are shown by the category Sealants.
May I touch up the joint after it has cured?
Material applied afterwards adheres only to a limited extent to the cured surface. It is better to cut the affected section out cleanly, prepare the flanks as in step 2 and seal again.
Questions about joint planning? We are happy to advise you: Baustoffheld, Lindenstrasse 56, 47623 Kevelaer, telephone +49 (0) 2831 - 916 77 03.
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