In our experience, many general purpose silicone sealants claim to be suitable for most common building materials. While this isn’t wrong, there are limitations to the adhesion of silicones on different materials.
The important note: silicone sealants can adhere to most materials, whether porous or non-porous. This includes glass, metal, wood, concrete, cement, stones, plastics etc. However the adhesive strength of silicone sealants can differ greatly among these materials.
There’s a general rule to this: silicone sealants have better adhesion to non-porous surfaces like glass and metal, and poorer adhesion to porous surfaces like wood and stone. This is not always 100% accurate (for example, plastics are non-porous but silicones do not adhere well to them), but it’s a good rule-of-thumb.
If there is no specific requirement for adhesive strength, it’s still possible to use silicones almost everywhere. However, if there are mechanical forces at work in the assembly, it may be better to use different sealant technologies if one or both of the substrates are porous.
The substrates where silicone sealants have the best adhesion to is glass and metal. This is why silicone sealants are so prevalent in window assemblies. It also has good adhesion to tiles and other bathroom fitting materials, which makes silicones a good choice in the bathroom.
For everything else, PU and hybrid polymer technologies would usually make a better choice. PU sealants have much greater adhesive strength to concrete, for example, but poorer adhesion to glass. In many ways, PU sealants and silicone sealants complement each other greatly, having good bonding strength to materials the other doesn’t.
However, at the top of the sealant food chain are still hybrid polymers, especially those based on Soudal’s SMX hybrid polymer technology. SMX polymers have greater adhesion than both silicones and PUs on a wider substrate spectrum. For most applications, you really can’t go wrong with hybrid polymers.
Still, for glass and metal, save your costs and stick to your tried-and-tested silicone sealants.