Silicone joint turning yellow – causes and solution

Short answer: In most cases a yellow silicone joint is not a material defect but a deposit on the joint – soap and grease film, lime soap, tobacco smoke or residues of care products. Discolouration like this can be cleaned off. If the discolouration is in the material, however (mould beneath the surface, outgassing from adjoining components, plasticiser migration), only a complete replacement of the joint helps. The fingernail scratch test tells both cases apart in seconds.

1. Soap, grease and lime-soap film

Shower gel, shampoo, hand soap and body oils combine with the lime in tap water and settle on the joint as a tough film. Over time this film yellows and is often mistaken for “broken silicone”. It is the most common cause – and the only one that can be fixed without changing the material.

2. Tobacco smoke and environmental influences

In its technical note on OTTOSEAL S125, OTTO Chemie expressly points out: “Heavy exposure to tobacco smoke and similar environmental influences can lead to discolouration of the sealant.” This affects every silicone – regardless of price and manufacturer. In kitchens, cooking fats and cooking vapours add to it.

3. Mould – first yellow, then black

Mould does not always show up black straight away. Incipient infestation often appears as a yellowish or reddish discolouration with blurred edges. The decisive point: once the mycelium has penetrated the joint surface, it can no longer be cleaned off. How to get infestation under control for good is explained in the guide Preventing and removing mould in silicone joints.

4. Water streaks from the smoothing agent

An application error that only becomes noticeable weeks later. OTTO Chemie writes: “When a smoothing agent is used, any water streaks that form must be removed immediately after sealing. If cleaning takes place at a later point, permanent streaks may remain.” These streaks pick up dirt and later show up as yellow longitudinal stripes.

5. Incompatible neighbouring materials

According to the manufacturer's note, the user must make sure that all materials in the contact area – solid, liquid and gaseous – are compatible with the sealant and do not discolour it. This also includes outgassing from materials installed later. Specifically, direct contact with bituminous and plasticiser-releasing materials such as butyl, EPDM, neoprene as well as insulating and black coatings is to be avoided. The plasticisers migrate into the silicone and discolour it permanently yellow to brownish.

Cut the joint out roughly with a sharp knife, then apply the Soudal silicone remover to the residues with the supplied brush, leave it to act for approx. 10 minutes and scrape it off with the spatula. The substrate must be dry for this – contact with water deactivates the active ingredients. The process can be repeated, but let it dry completely in between.

Check: Run your fingernail along the joint flanks. Nothing rubbery may lift off any more. Described in detail in the guide Removing old silicone without residue.

If the old joint was infested with mould, the affected areas must be treated with anti-mould spray before re-jointing to eliminate any remaining fungal spores. OTTO Chemie is clear on this: without this step, infestation can return very quickly despite the fungicidal finish of the new sealant.

The bonding surfaces must be clean, free of grease, dry and sound. Clean non-porous substrates such as tiles with OTTO Cleaner T and a clean, lint-free cloth – flash-off time approx. 1 minute. Clean porous substrates of loose particles mechanically, for example with a steel brush or grinding disc.

Application temperature +5 °C to +35 °C. Dispense the cartridge evenly with a manual, compressed-air or battery-powered gun and smooth it before skin formation with smoothing agent and a finishing spatula.

Critical point against later yellowing: Remove water streaks from the smoothing agent immediately after sealing. Later, permanent streaks remain. Fresh silicone residues can be taken off with white spirit before curing, afterwards only with silicone remover or mechanically.