What is the difference between sanitary silicone and general purpose silicone?

Three variables hide behind the labels on a silicone cartridge and only one of them is usually on the front. There is the base polymer, which is much the same across the range; there is whether a fungicide has been added; and there is the cure chemistry, which decides what the sealant does to the surface it is stuck to while it cures. That third one causes more expensive mistakes than the other two together.

Safety

Step by step

1. Check the cure chemistry first.Acetic cure smells strongly of vinegar and releases acetic acid. Neutral cure — sometimes labelled oxime or alkoxy — smells mild. Acetic grades are cheaper, cure faster and bond well to glass and glazed tile. Neutral grades are essential on mirrors, natural stone, copper, brass, fresh concrete, and some plastics and painted surfaces.
2. Then decide whether you need the fungicide.Sanitary grades contain a biocide that slows mould colonisation. Anywhere permanently damp — a shower, a bath, a basin, a kitchen sink surround — it is worth the small extra. In a dry internal joint it does nothing useful.
3. Understand what the fungicide does not do.It protects the silicone itself. Mould in a bathroom mostly grows on the soap film, body oils and shampoo residue lying on top of the bead, and the fungicide does nothing about that. A sanitary sealant kept covered in soap scum will still go black.
4. Check the substrate before anything else.Marble, limestone, granite, a mirror, bare copper pipe, brass, fresh mortar, polycarbonate. On any of those, the decision is made: neutral cure, regardless of what the front of the tube emphasises.
5. Look at the movement capability if the joint moves.Stated as a percentage. Sanitary grades are usually low modulus with good movement capability precisely because a bath or a tray moves as it is loaded and emptied. A stiff general-purpose grade in that joint splits sooner.
6. Do not choose on colour alone.White, clear, grey, black, brown and various tile-matched shades exist across both types. Choosing on colour first and grade second is a common way to end up with an acetic general-purpose product on a stone worktop.
7. Check the date on the cartridge.Silicone has a shelf life of roughly twelve to eighteen months and out-of-date stock is one of the commonest reasons a bead stays soft. The date is stamped on the cartridge, not always on the label.

Tips

• If the tube does not say what its cure chemistry is, smell the nozzle after piercing it. A sharp vinegar smell means acetic and means it should stay away from stone, mirrors and copper.
• Neutral cure takes noticeably longer to skin and to cure through — often twice as long — which is a real consideration in a bathroom you need to use.
• Clear silicone shows every fault in the joint behind it. White or a matched colour is more forgiving on anything less than perfect.
• A sanitary grade is not a substitute for ventilation. The extract fan does more for the life of a bathroom bead than the grade of sealant does.
• Aquarium-grade silicone is a neutral, biocide-free product, and it is the one to use where fish or plants are involved, because ordinary sanitary fungicide is toxic to them.

Common mistakes

• Using acetic silicone on a stone worktop or windowsill — The acid etches marble, limestone and some granites permanently, leaving a dull mark along the joint that cannot be polished out.
• Using acetic silicone against bare copper or brass — It corrodes both. On a pipe passing through a sealed joint this is a slow leak in waiting.
• Assuming sanitary grade prevents mould — It slows growth in the sealant. The mould you actually see is usually growing on the soap film on top of it, which the fungicide never touches.
• Buying general purpose because it was cheaper, for a shower — It has no fungicide and often less movement capability. In a wet joint it blackens sooner and splits sooner, and redoing it costs a whole evening.

Questions people ask