The short answer: In six steps – clean and degrease, fill behind, cut the cartridge and trim the nozzle to the joint width, apply in one pass under even pressure, smooth immediately before a skin forms, and let it cure. Two mistakes decide the result: smoothing too late – silicone forms a skin after 10 to 30 minutes – and too much smoothing agent, which gets between sealant and flank and prevents adhesion there. Application temperature 5 to 40 °C. The joint is loadable after about 24 hours, cured through at around 2 mm per day.
What you need
- The right silicone – sanitary, construction, natural stone or high-temperature, depending on the use
- Cartridge gun with anti-drip mechanism – without it the mass runs on and the joint becomes uneven
- Utility knife or cartridge opener
- Closed-cell PE round cord to match the joint width
- Joint smoother in several radii or a smoothing-tool set
- Spray bottle with smoothing agent – water with washing-up liquid will do in a pinch, but is harder to dose
- Isopropanol or white spirit and lint-free cloths
- Masking tape if clean edges are called for
- Disposable gloves – cured silicone comes off the skin only mechanically
Step 1: prepare the substrate
This is where it is decided whether the joint lasts two years or ten. Both flanks must be dry, free of dust and free of grease and free of loose parts, paint residue, rust and release agents.
Old joints are removed completely – fresh silicone does not adhere to cured silicone. Work with a joint knife or plastic scraper, not with the cutter: in waterproofed bathrooms the blade cuts through the waterproofing beneath the tiles, and on acrylic bathtubs it leaves scratches. Let residue swell with a silicone remover and push it off.
For degreasing use isopropanol or white spirit, not washing-up liquid – surfactants leave a film themselves. With sanitary acrylic there is the added point that a factory wax layer lies on the surface, which has to be rubbed off; why that goes wrong so often is explained in our guide Silicone on acrylic bathtubs.
Absorbent substrates such as concrete, plaster or natural stone additionally need a primer. It does not replace the cleaning but comes after it.
Step 2: fill behind
The step that is skipped most often and achieves the most. A closed-cell PE round cord is pressed into the joint – diameter around 25 percent larger than the joint width, so that it wedges.
It has two tasks: it sets the joint depth, and it prevents adhesion at the back of the joint. If the sealant additionally adheres to the back of the joint, it is held from three sides when stretching and tears – no matter how good the product is.
Press the cord in with a blunt tool, never with a screwdriver: a damaged closed-cell cord gives off gas, and bubbles form in the fresh joint. If the joint is too shallow for a cord, a PE separating tape takes on the same task. Which dimensions apply for what is explained under How wide may a silicone joint be.
Step 3: prepare cartridge and nozzle
Cut off the thread above the cartridge neck, then screw on the nozzle. Anyone who cuts too deep can no longer get the nozzle to seal on the thread.
The nozzle is cut at a slant and to the width of the joint – better a little too small than too large, you can always cut more. The slanted cut ensures that the nozzle lies flat in the joint and the mass is pressed backwards, instead of lying on top.
Insert the cartridge into the gun and advance the plunger until it makes contact. In the cold silicone is tough: a cartridge that has stood in the car or in an unheated store should lie at room temperature for a few hours beforehand.
Step 4: apply
Place the nozzle at an angle of about 45 degrees and push, do not pull – that way the mass is pressed into the joint instead of merely running over it.
Even pressure and even speed are more important than pace. Ideal is one continuous pass over the whole section; every stop creates a spot where a notch or air pocket sits later.
Work in sections of one to two metres and smooth each section immediately. Anyone who first applies ten metres and then begins to smooth already has a skin at the start.
At the end of the section operate the release lever of the gun, otherwise the mass runs on.
Step 5: smoothing – the decisive moment
Depending on product and room climate, silicone forms a skin after 10 to 30 minutes. After that it can no longer be smoothed cleanly; the smoother tears the surface open instead of shaping it.
Spray smoothing agent onto the smoother, not into the joint. One spray is enough. Too much agent runs onto the flanks and prevents there exactly the adhesion you need – a mistake you see only months later, when the joint comes away on one side.
Draw over the section in one pass with light, even pressure. The smoother forms the fillet and at the same time presses the mass against the flanks. Wipe excess off the smoother regularly, otherwise you smear it back into the joint.
The finger is usable as an emergency solution, but worse: it leaves an irregular fillet, and the warmth of the hand accelerates skin formation. If so, then with a glove and dampened.
If masking tape was used, it is pulled off immediately after smoothing – at a slant outwards, away from the joint. Anyone who waits until a skin has formed tears the fresh edge off with it.
Step 6: let it cure
Silicone cross-links with the humidity of the air from the outside in, at about 2 mm per day. The surface is dry after an hour, the core of a joint 8 mm deep needs several days.
In practice that means: loadable after around 24 hours, longer in cool or very dry air. During this time do not put the joint under water load and ventilate the room – moisture in the air accelerates cross-linking, standing water on the joint disturbs it.
Removing silicone properly when something goes wrong
Fresh silicone can be taken off with a dry cloth – not smeared, but lifted off. Work over residue with white spirit or isopropanol while it is still soft.
Silicone that has started to harden is the unpleasant intermediate state: too firm for wiping, too soft for pulling off. Here the only remedy is to wait for it to cure.
Cured silicone comes off only mechanically. Cut it off with a plastic scraper or joint knife, let the remaining film swell with a silicone remover and push it off. On delicate surfaces – acrylic, varnished wood, natural stone – test beforehand at a concealed spot. Our guide Removing old silicone without leaving residue describes this in detail.
Difficult situations
The corner
Internal corners are the spot at which most joints turn out untidy. The trick: do not run through, but apply both legs separately and work each up to just before the corner. Then smooth one leg first, letting it run out beyond the corner, then the second. Anyone who tries to get round the corner in one pass presses the mass up in the angle.
Vertical joints
Here gravity works against you. Apply from the bottom up, so that the mass is pressed into the joint instead of running out, and work in shorter sections. A product set to be non-sag is considerably more pleasant on vertical joints than a soft one.
Long joints without a break
Over several metres one cartridge is seldom enough. Plan the change: have the second cartridge ready prepared, make the transition in the middle of a section and carry on working immediately – not at a corner or a visible point. When smoothing, the transition then becomes invisible.
Overhead
Connections at the ceiling are unpleasant, because the mass drips and smoothing agent runs into your face. Put on safety glasses, choose a smaller nozzle opening, short sections, and wipe off the smoother more often than usual.
Storage and shelf life
Unopened cartridges keep, stored cool and dry, for about twelve to eighteen months; the date is on the base of the cartridge or the seam. Storage that is too warm, above 30 °C, makes the material age faster and become tough, whereas frost does not permanently harm most silicones – the cartridge only has to come back to room temperature before application.
An opened cartridge keeps only for a limited time. In the nozzle the mass cures and forms a plug; at the next use it can usually be pulled out, but the material underneath has often already begun to harden. Anyone working over several days screws on a fresh nozzle and closes it airtight – a piece of adhesive tape over the opening is enough.
A note on disposal: cured silicone residue belongs in residual waste, not down the drain. Liquid residue is hazardous waste.
Which silicone for which use?
| Use | Product group | What to watch for |
|---|---|---|
| Bathroom, shower, kitchen | Sanitary silicone, neutral | with fungicidal additives |
| Natural stone, marble | Natural stone silicone | no migrating plasticisers |
| Windows, glazing beads | Window or construction silicone | UV-resistant, weatherproof outdoors |
| Wooden windows | neutral or hybrid | Acetoxy silicone attacks coatings |
| Metal, galvanised, copper | neutral-curing | Acetic acid causes corrosion |
| Aquarium, permanent wetness | Special silicone | express approval needed |
| Stove, fireplace, flue gas | High-temperature silicone | check the temperature range |
| Joint will be painted over | no silicone | Take acrylic or hybrid |
The most frequent mistakes
By far the most frequent mistake. After skin formation the smoother tears the surface open. Work in short sections and smooth immediately.
It gets between sealant and flank and prevents adhesion there. Spray sparingly onto the smoother, never into the joint.
Adhesion at the back of the joint, a stiff cross-section, unnecessary material usage – and a joint that tears exactly where it should be working.
Surfactants leave a film. Use isopropanol or white spirit and let it flash off completely.
Fresh silicone does not adhere to cured silicone. The new joint comes away after a few weeks together with the old.
After skin formation the joint edge tears off with it. Pull off immediately after smoothing, at a slant outwards.
Frequently asked questions
How do you apply silicone correctly?
Clean and degrease, fill behind, cut the nozzle to the joint width, apply in one even pass, smooth immediately and let it cure for 24 hours. What is decisive is the preparation and the moment of smoothing.
How long does silicone have to dry?
Skin formation after 10 to 30 minutes, loadable after about 24 hours, complete curing through at around 2 mm per day. A deep joint correspondingly needs several days.
What is the best way to smooth silicone?
With a joint smoother of the right radius and a smoothing agent sprayed sparingly onto the tool. The finger will do in a pinch, but gives an irregular fillet and accelerates skin formation.
Can silicone be applied without smoothing agent?
Yes, water with a drop of washing-up liquid works. It is only harder to dose, and too much of it disturbs adhesion at the flanks – with a ready-made product the quantity is easier to control.
At what temperature can silicone be applied?
Between about 5 and 40 °C. Below that the mass becomes tough and hardly cross-links, in blazing sun it forms a skin too quickly and can no longer be corrected.
How much silicone do I need?
Width times depth times length gives the usage in millilitres. A joint 8 × 8 mm uses around 64 ml per metre, so a 310 ml cartridge is enough for about 4.8 metres. Exact values per joint dimension are in our usage table.
Can you paint over silicone?
No. Paint beads off or cracks the first time the joint moves. Where painting over is intended, acrylic or a paintable hybrid sealant belongs there – what properties these sealants bring is explained in the article on MS polymer sealants.
Why do I get bubbles in the joint?
Three causes. Usually the closed-cell round cord was pierced when pressing it in and gives off gas. Second possibility: during application there was a stop and air was trapped. Third: the substrate was still damp. All three show only after curing – prevention is the only way.
Do you have to mask when sealing?
It is not necessary, helpful with visible edges, on rough plaster and everywhere a straight line is called for. The moment of pulling off is more important than the question itself: immediately after smoothing, at a slant outwards. Anyone who waits tears the edge off with it.
How do I get an even joint width?
Via the nozzle, not via the hand. A nozzle cut to suit, lying flat in the joint, limits the width by itself. Add even speed – anyone who slows down applies more.
Can silicone be applied to a damp substrate?
No. The substrate must be dry, otherwise the sealant does not adhere and bubbles form. If cleaning was done wet, the joint has to dry off completely – with natural stone and absorbent substrates that can take longer than you think.
How do I open a silicone cartridge correctly?
Cut off the thread above the cartridge neck, not below it – otherwise the nozzle no longer seals. Then screw on the nozzle and trim it at a slant to the joint width. With aluminium cartridges unscrew the closure cap and pierce the foil.
