Silicone, PU or Acrylic Sealant? Choosing the Right Sealant for the Joint
Choose the sealant by the joint, not the chemistry. A crack in plaster or a skirting joint that will be painted takes an acrylic sealant such as Akfix AS606. A concrete floor, slab or precast panel joint that moves takes a low-modulus polyurethane (PU) such as Akfix P635. A gutter, mabati seam, duct or container body takes a high-modulus PU metal sealant such as Akfix P645. Anything permanently wet, and anything involving glass, takes silicone: an acetoxy silicone such as Akfix 100E indoors, and a neutral-cure silicone such as Akfix 915N or KNICKER 995 outside and on metal or stone. One rule settles most arguments on site: silicone cannot be painted over.
The decision table: start from the joint
| The joint | Use | Why |
|---|---|---|
| Crack in plaster wall or ceiling, skirting, cornice, frame-to-wall gap that will be painted | Akfix AS606 acrylic | Paintable, water-based, handles movement up to 15%. Not for joints permanently exposed to water. |
| Expansion joint in a floor or slab, joint between precast concrete panels, connection joint between different building materials | Akfix P635 PU construction sealant | Low modulus, 25% movement capability, published for pedestrian and traffic areas, paintable. |
| Gutter, downpipe, mabati or sheet-metal seam, ventilation duct, container or truck body | Akfix P645 PU metal sealant | High modulus, bonds and seals metal against water, air and dust, over-paintable. |
| Shower tray, bath, basin, kitchen worktop on tile, glass or porcelain (indoors) | Akfix 100E acetoxy silicone | Stays elastic, published for bathroom, kitchen and plumbing joints. |
| Window and door perimeters outside, facade joints, marble and stone, aluminium frames | Akfix 915N or KNICKER 995 | Neutral cure, non-corrosive, made for weather sealing and glazing. |
If the joint does not fit a row, ask two questions. Will it be painted? Then it is acrylic or PU, never silicone. Will it be wet all the time? Then it is not acrylic.
Acrylic: painted walls and low-movement joints
Akfix describe AS606 as a one-component acrylic emulsion sealant reinforced with silicone emulsion. It is water-based, waterproof once cured and over-paintable, for porous surfaces such as brick, concrete and wood.
The limits come from Akfix too: movement up to 15%, joint widths of 5 to 25 mm, and not for joints permanently exposed to water. Keep the sealed joint dry for at least two hours, and prime porous surfaces first as the data sheet describes. It costs KSh 450 for a 300 ml cartridge. It is listed in our acrylic sealants collection.
Polyurethane: two grades, two different jobs
Both Akfix PU sealants cure with atmospheric moisture and stay elastic. The difference is the modulus: as a general rule, how stiff the cured sealant is.
P635: low modulus, for joints that move
Akfix P635 has a 25% movement capability and is published for expansion joints between construction materials, movement and connection joints in floors, joints between prefabricated elements and expansion joints between precast concrete panels, indoors and outdoors, including pedestrian and traffic areas. Akfix state it does not shrink and is paintable. Akfix specify a 2:1 width-to-depth ratio with a backer rod, so the sealant bonds to two sides only. It comes as a 600 ml sausage at KSh 1,450, or KSh 16,200 for a box of 12.
P645: high modulus, for metal
Akfix P645 adheres to sheet iron, aluminium, stainless steel, lead, copper, ceramic, glass, wood and various plastics. Akfix publish it for sheet-metal seams, gutters, ducts and container and vehicle bodies. It has high mechanical resistance and is over-paintable, with a service range of -40 °C to +90 °C. It is a 310 ml cartridge at KSh 950, or KSh 10,800 for 12. For movement joints in concrete, use P635 instead.
Silicone: wet areas and glass
Silicone is the right answer where water sits on the joint or where glass is involved, and the wrong answer anywhere that will be painted. Akfix 100E is an acetoxy silicone for bathrooms, kitchens, plumbing and joints on glass, porcelain and steel, at KSh 450 a 280 ml cartridge or KSh 9,600 for 24. Outside, and on metal or stone, use a neutral cure: Akfix 915N (600 ml, KSh 950, or KSh 10,800 for 12) is made for weather sealing windows and facades and for marble and stone; KNICKER 995 (600 ml, KSh 500, or KSh 9,000 a carton) is non-corrosive to coated glass, marble, concrete and metal other than copper. For everyday sealing on a budget, ABC GP fast-curing (280 ml, KSh 170, or KSh 3,840 a carton) is a general-purpose silicone for doors, aluminium windows, tiles and cabinets, indoors and out.
Choosing between silicones is a separate question, covered in which silicone sealant to use and structural vs ordinary silicone. Every grade is in our silicone sealants collection.
What each one costs per metre
Cartridge price tells you little, because pack sizes run from 280 ml to 600 ml. The table divides today's single-piece price by the joint length per pack given on each product page, before waste. The first four are compared at the same 15 x 8 mm joint; 100E and ABC GP only have figures for smaller joints, so they are shown at 6 x 6 mm.
| Product | Pack and price | Joint | Length per pack | About KSh per metre |
|---|---|---|---|---|
| Akfix AS606 acrylic | 300 ml, KSh 450 | 15 x 8 mm | 2.5 m | 180 |
| Akfix 915N neutral silicone | 600 ml, KSh 950 | 15 x 8 mm | 5 m | 190 |
| Akfix P635 PU construction | 600 ml, KSh 1,450 | 15 x 8 mm | 5 m | 290 |
| Akfix P645 PU metal | 310 ml, KSh 950 | 15 x 8 mm | 2.6 m | 365 |
| Akfix 100E acetoxy silicone | 280 ml, KSh 450 | 6 x 6 mm | 7.8 m | 58 |
| ABC GP fast-curing silicone | 280 ml, KSh 170 | 6 x 6 mm | 7.8 m | 22 |
Buying by the box brings P635 down to about KSh 270 a metre and P645 to about KSh 346 at 15 x 8 mm. PU costs more per metre, but on a moving slab or a gutter seam the cheaper products are the wrong material. Joint size changes the cost too: P635 at 20 x 10 mm runs 3 metres a sausage, about KSh 483 a metre.
Getting the joint right
The Akfix data sheets for P635 and 915N both give a width-to-depth ratio of about 2:1. For the full method, backer rod sizing and tooling, see how to seal expansion joints.
Where to buy
All of these are in our sealants collection, with the Akfix data sheets attached to the AS606, P635, P645, 100E and 915N product pages. Collect from our Nairobi trade counter at Royal Business Park, opposite Complast, Old Mombasa Road, Monday to Saturday, 8:00 to 17:30, with free pickup, or order for countrywide delivery at a flat KSh 350, free on orders over KSh 15,000. Contractors and hardware shops buying by the box can request a trade quote on our trade page. Not sure which joint you have? Send a photo on WhatsApp.
Common questions
Can I paint over silicone?
No. As a general rule, paint does not hold on cured silicone. If the joint will be painted, use AS606 acrylic on low-movement joints or P635 or P645, which Akfix state are paintable.
Is polyurethane better than silicone?
Neither is better in general. PU suits concrete movement joints and metal seams, and can be painted. Silicone suits wet areas, glass and window perimeters. Pick by the joint using the table above.
Can I use acrylic sealant in a bathroom?
Not on joints that stay wet. Akfix say AS606 must not be used on joints permanently exposed to water. Around a shower tray or basin, use a silicone such as 100E.
What sealant do I use for a concrete floor joint?
A low-modulus PU such as Akfix P635, which Akfix publish for movement and connection joints in floors and for pedestrian and traffic areas, set to depth on a backer rod.