Which silicone sealant should I use? Bathroom, kitchen, window and glass

For a bathroom, kitchen or around a basin, a general-purpose acetic (acetoxy) silicone is fine on tile, glass and ceramic. For mirrors, natural stone, metal frames, concrete and any outdoor window or facade joint, use a neutral-cure silicone, because acetic silicone gives off acid as it cures and can damage those surfaces. If the joint will be painted, or it is a crack in a plastered wall, use an acrylic sealant instead of silicone.

The rest of this guide covers the four types on a Kenyan hardware shelf, joint preparation, bead size and the mistakes that make silicone peel.

The four types of sealant, in plain terms

1. Acetic (acetoxy) silicone: the everyday general-purpose tube

You can recognise it by the vinegar smell while it cures. It skins quickly, grips well on glazed tile, glass, porcelain and enamel, and stays flexible. It is what most fundis mean when they say "silicone".

Two examples we stock: Akfix 100E Universal Silicone and Pattex SL212 General Purpose Silicone, both acetoxy and both in 280 ml cartridges. Akfix publishes 100E for bathroom, kitchen and plumbing joints, window and door sealing and joints on glass, porcelain and steel.

The limits matter more than the uses. Akfix's data sheet for 100E says it must not be used on mirrors or on copper, brass or lead, is not recommended on porous surfaces such as marble, natural stone and concrete, and is not paintable. Henkel says the same for SL212: not against lead, copper, zinc or iron, not on mirrors or aquaria, not for drinking-water contact and not on acrylic (PMMA) sheet, which rules out most acrylic shower screens.

2. Neutral-cure silicone: windows, facades, stone and metal

Neutral silicones cure without releasing acid, have very little smell and are non-corrosive. That makes them the right choice for aluminium windows, external perimeter joints, marble and granite, concrete and anywhere a metal could stain or corrode.

Akfix 915N Weatherseal is a neutral, low-modulus silicone made for weather sealing and glazing; its data sheet gives a skin time of 5 to 10 minutes and a cure rate of at least 2.5 mm per day at 23 °C and 50% humidity, and recommends joint widths between 10 mm and 35 mm. The 590 to 600 ml "structural" sausages on our shelf, such as KNICKER 995 Structural Sealant and ABC GP Structural Silicone, are also neutral-cure and are bought mainly by aluminium and glazing fabricators.

On a true structural glazing job, where the silicone holds the glass on a curtain wall, follow the facade engineer's specification, not the shelf label.

3. Mirror silicone: a neutral grade made for mirror backing

Mirror backing paint is easily attacked by the acid in acetic silicone, which shows up later as dark spots creeping in from the edge. Use a neutral mirror adhesive such as Akfix 900N. We cover the method in detail in our guide on how to fix a mirror to a wall without drilling.

4. Acrylic sealant: paintable, for cracks and interior joints

Acrylic sealant is water-based, cleans up with water while wet and can be painted. It is the right product for gaps between door frames and plaster, skirting joints and hairline cracks in walls and ceilings. It is not a wet-area product. Akfix AS606 Siliconized Acrylic handles movement up to 15%, but Akfix says not to use it on joints permanently exposed to water and to keep the joint dry for at least two hours after application.

Which one for which job

• Shower tray, bath, basin and kitchen worktop joints (tile, ceramic, glass): acetic silicone, white or clear. Use neutral if there is natural stone, a metal trim or a mirror nearby.
• Granite or marble kitchen tops: neutral silicone. Acetic silicone is not recommended on natural stone.
• Aluminium windows and doors, inside and out: neutral silicone such as 915N or a structural sausage.
• External facade, expansion and perimeter joints: neutral, low-modulus silicone with a backer rod.
• Mirrors: neutral mirror adhesive only.
• Cracks in plaster, gaps at door frames and skirting that will be painted: acrylic sealant.
• Gutters, sheet metal seams and ducting: look at a polyurethane sealant such as Akfix P645 PU Metal Sealant, which Akfix publishes for those jobs and which can be painted.

How to prepare the joint

1. Remove old sealant completely. New silicone does not stick to old silicone. Cut it out, then scrape and wipe off the film it leaves.
2. Clean and dry. Akfix's data sheets recommend degreasing glass, aluminium and stainless steel with alcohol or MEK, lightly abrading other metals, lightly sanding wood and brushing dust off concrete. The joint must be dry; silicone will not bond to a wet tile edge.
3. Mask the edges with tape for a straight line.
4. Use a backer rod in deep joints. For wide joints, push in a foam backer rod so the sealant sits at the right depth and only sticks to the two sides, not the back.
5. Check the temperature. Akfix gives +5 °C to +40 °C for its silicones.

Bead size: how wide and how deep

A sealant joint should be wider than it is deep. Akfix's data sheets give a width-to-depth ratio of about 2:1 and a minimum joint of 5 mm by 5 mm. For 100E, they recommend a 6 mm depth for joints between 6 mm and 12 mm wide. A deep, narrow bead is stiff and tears; a shallow, wider bead flexes with the joint.

To estimate how far a cartridge goes, divide its volume by the bead cross-section. A 6 mm by 6 mm bead is 36 square millimetres, which is 36 ml per metre of joint. A 280 ml cartridge therefore covers about 280 ÷ 36 = 7.8 metres before waste. A triangular bead in a bath corner with 6 mm legs is half that cross-section, so the same cartridge runs to roughly 15 metres. See silicone sealant prices in Kenya for what that works out to per metre.

Application and cure

• Cut the nozzle at 45 degrees to slightly smaller than the joint width, so you can push the bead in rather than lay it on top.
• Push the gun forward along the joint in one steady pass.
• Tool the bead straight away. Akfix says to tool 100E and 915N with light pressure within 5 minutes; Henkel gives SL212 a skin time of about 15 minutes. Once a skin forms, tooling drags and tears it.
• Leave it alone. Silicone cures from the surface inwards, roughly 2 to 3 mm per day for the products above under standard conditions, so a 6 mm joint needs two to three days to cure through. Keep a new shower joint dry for at least 24 hours, longer if you can, and check the cartridge for the manufacturer's figure.
• Work with ventilation and keep uncured silicone off skin and out of eyes. Cool or damp weather slows the cure, so allow extra time before the joint is used.

Common mistakes

• Acetic silicone on a mirror. A common cause of black edges on bathroom mirrors.
• Sealing over old, mouldy silicone. It will peel, and the mould comes through.
• Trying to paint silicone. Paint will not hold. Use acrylic where the joint will be painted.
• Acetic silicone on marble or granite. It is not recommended for porous natural stone and can leave a mark.

Buying in Nairobi

You can compare every silicone we hold, with colours, cartridge sizes and carton prices, in our silicone sealants collection. Contractors and hardware shops buying by the carton can request a trade quote on our trade page. Our Nairobi trade counter is at Royal Business Park, opposite Complast, Old Mombasa Road, open Monday to Saturday, 8:00 to 17:30, and we deliver countrywide for a flat KSh 350, free on orders over KSh 15,000. If you are not sure which grade your job needs, send a photo of the joint on WhatsApp.

Common questions

Is acetic silicone waterproof enough for a shower?

Yes, on glazed tile, ceramic and glass. Henkel describes SL212 as waterproof and mildew resistant. It is not for permanent immersion, such as inside an aquarium or a water tank.

How do I tell if a silicone is neutral or acetic?

Read the label or data sheet for "acetoxy", "acetic", "neutral" or "oxime". If there is nothing written, a strong vinegar smell when you squeeze some out means it is acetic.

Can I use silicone on a wall that I will paint later?

No. Silicone is not paintable. Use an acrylic sealant such as AS606 for joints that will be painted.