Silicone joint will not harden – causes and solution
A silicone joint usually does not cure through because work was done below +5 °C, the joint is too thick or too deep without a backing profile, or the material had expired or came into contact with incompatible substances. Silicone cures from the outside in, as a rule of thumb about 2–3 mm per 24 hours. If the joint stays soft or tacky after several days, usually only complete renewal under the right conditions helps.
Contents
What you need
Project template “Renew the silicone joint in the bathroom”A silicone joint usually fails to cure through because the work was done too cold (below +5 °C), the joint was applied too thick or too deep without a backing profile, or material was used that had expired or came into contact with incompatible substances. Freshly applied silicone at first forms only a thin skin – curing right through takes considerably longer depending on the joint thickness, with a rule of thumb of about 2–3 mm per 24 hours. If the joint still feels noticeably soft or tacky after several days, usually the only remedy is to renew it completely under the right conditions.
%20Otto%20Chemie.webp?ts=1782992979)
Why does silicone sometimes not cure?
Silicone sealants such as OTTOSEAL S100 or S110 are 1K systems that cross-link from the outside in through the humidity of the air. If a joint stays tacky or completely soft, in practice mainly four causes come into question:
Application temperature too low
Ottoseal silicones are designed for an application temperature of +5 °C to +35 °C. At wintry temperatures or in unheated rooms the cross-linking reaction slows down greatly or comes almost completely to a standstill.
Joint too thick or too deep
Silicone cures from the outside in, on average about 2–3 mm per 24 hours. According to the manufacturer, the vulcanisation time lengthens with increasing layer thickness – a deep joint without a backing profile can stay soft at the core for weeks.
Contact with incompatible materials
Contact with bituminous or plasticiser-releasing substances such as butyl, EPDM, neoprene or black paints can disturb the cross-linking and keep the joint permanently tacky.
Expired or wrongly stored material
Unopened cartridges can be stored cool and dry for 18 to 24 months, depending on the product. After the best-before date has passed, or if stored in the warmth, cross-linking can be incomplete.
Solution 1: the joint is only a few days old and tacky only on the surface
Directly after sealing, at first only a skin forms – that is normal and no sign of a problem. Until the joint has also cured through at the core, a little patience is needed depending on the width:
As long as the surface is still sticky, the joint should not be showered on, cleaned or put under mechanical load.
With a usual joint 5–8 mm wide, complete curing at the core can therefore take several days.
Temperatures above +5 °C and normal humidity in the room ensure good curing through – when Ottoseal S110 cures, small amounts of an oxime compound are also released, so ventilate well during curing.
If the joint still feels noticeably soft or tacky after about a week, one of the causes named above is as a rule present – then there is no way round renewing it.
Solution 2: the joint stays permanently soft or tacky – renew it completely
If a joint has not cured through even after sufficient time, the only remedy is complete removal and resealing under controlled conditions. With deep joints a backing profile is decisive here, so that only a thin, well-curing layer of silicone is applied:
| Step | What to do | Check |
|---|---|---|
| 1. Remove the soft silicone | Remove the joint that has not cured through completely with a cutter or joint scraper, including remains on the flanks. | No silicone residue visible or detectable by touch |
| 2. Check the joint depth | For joints deeper than approx. 10 mm insert a PE round cord joint filling profile – water-repellent and closed-cell to DIN 18540, in diameters from 6 to 40 mm. | Joint depth reduced, silicone lies on it only thinly |
| 3. Clean the substrate | Make the contact faces free of grease, clean and dry. | Substrate feels free of grease and grippy |
| 4. Reseal at the right temperature | Apply only between +5 °C and +35 °C. Use OTTOSEAL S100 for sanitary areas, OTTOSEAL S110 for construction and window connection joints. | Even bead of sealant without air pockets |
| 5. Smooth before a skin forms | Smooth with smoothing agent and joint smoother, e.g. with the OTTO joint smoother spatula. | Smooth, closed surface |
| 6. Allow sufficient time to cure through | Observe the rule of thumb of 2–3 mm per 24 hours, and do not wet or load the joint in this time. | Joint feels completely firm (scratch test at an inconspicuous spot) |
You will find detailed step-by-step instructions with pretreatment and tools in the How to: replace sanitary silicone – step by step. You can work out the material needed in advance with the Ottoseal silicone usage table.
How to avoid the problem next time
Keep to the application temperature
Seal only between +5 °C and +35 °C – this applies to storing the cartridge shortly before use as well.
Limit the joint depth
With deep joints insert a backing profile so that no more silicone is introduced than necessary.
Use compatible materials
Avoid contact with bituminous or plasticiser-releasing materials, especially with construction and window connection joints.
Use fresh material
Store cartridges cool and dry and use them within the best-before date printed on them.
Common mistakes – and how to avoid them
Joint applied too thick in one go
Better to limit the joint depth with a backing profile than to run a very thick joint that stays soft at the core.
Sealed in the cold
Application below +5 °C stops the cross-linking almost completely – observe the application temperature and heat the room if necessary.
Deep joint filled without a backing profile
Without a round cord the core of the joint often stays tacky for weeks, even if the surface seems firm long since.
Joint loaded or wetted too early
Moisture or pressure on silicone that has not yet cured through prevents clean curing – allow sufficient time.
Frequently asked questions
How long does it take for silicone to cure through completely?
As a rule of thumb, about 2–3 mm per 24 hours applies, depending on humidity and temperature. With a usual joint 5–8 mm wide, complete curing at the core can therefore take several days.
Why does my silicone joint stay tacky?
Usually it is down to an application temperature that was too low, a joint applied too thick without a backing profile, contact with incompatible materials, or expired material.
Can I simply let a tacky joint cure further, or do I have to remove it?
If the joint is only a few days old and tacky only on the surface, a little patience at a sufficient temperature is often enough. If it still stays soft after about a week, usually the only remedy is complete renewal.
What temperature does silicone need to cure?
Ottoseal silicones such as S100 and S110 are designed for an application temperature of +5 °C to +35 °C. Below that the cross-linking slows down or stops.
What is a backing profile and why do I need one?
A backing profile such as the PE round cord limits the joint depth, so that only a thin layer of silicone is applied. That speeds up curing and prevents unwanted adhesion at the back of the joint as well as at the flanks.
Can expired silicone be the reason it does not harden?
Yes. Unopened cartridges can be stored cool and dry for 18 to 24 months, depending on the product. After the best-before date has passed, cross-linking can be incomplete.
Related topics: Guide: Preventing and removing mould in silicone joints and Guide: Silicone joint keeps cracking – causes and solution. You will also find suitable products directly in our category sanitary silicone.
More guides on this topic
All guides on “Problems with silicone joints”
Instructions
Guide: cleaning silicone joints – what helps and what harms
Clean silicone joints properly: scratch test, household remedy check and deep clean
Problem solving
Guide: preventing and removing mould in silicone joints
Remove mould from silicone joints, when the joint has to be renewed and how to prevent it
Problem solving
Guide: silicone joint is pulling away from the wall – causes and fix
Why silicone joints come away from tile, bath or frame and how the new joint holds