Silicone sealants
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Frequently asked questions
How do silicone sealants differ from RTV gasket makers?
Both are room-temperature curing silicones, but general silicone sealants are formulated for joint filling and weatherproofing on glass, aluminium and cabinets. RTV gasket makers add oil and higher heat resistance for machine flanges. Building-grade silicone used as an engine gasket tends to leak early.
Are there silicones for food contact areas?
Yes, food-contact silicones are typically certified to FDA 21 CFR 177.2600 or NSF 51. They are usually neutral cure and free of toxic fungicides. Ask for the matching certificate for the specific product code.
Why won't paint stick to silicone sealant?
Silicone has low surface energy and bleeds silicone oil to its surface, so most paints peel. If overpainting is needed, use an MS polymer or polyurethane sealant instead. Silicone can also contaminate nearby powder coating areas.
What joint dimensions should a sealant bead have?
Joints are usually filled to a depth of about half the width, minimum 6 mm, using backer rod to control depth. The sealant should bond to the two sides only, not the bottom, so it can move freely. Over-deep beads cure very slowly at the core.
How should a joint be prepared before applying silicone?
The joint must be clean, dry, dust-free and clear of old silicone, since fresh silicone does not bond to cured silicone. Wipe glass, metal and plastic with isopropyl alcohol. Masking both edges of the joint gives a neat bead that is easy to tool.
How long does silicone take to skin and fully cure?
Silicone skins over in about 5 to 30 minutes, but cures inward only about 2 to 3 mm per day under normal conditions. A thick bead takes several days to cure through. Cold, dry weather slows this further.
Why does silicone go black with mould, and what can be done?
Mould grows on joints that stay damp with organic residue, common in washrooms and wash-down areas. Fungicide grade silicone limits it but cannot prevent it completely. Once mould penetrates the sealant, cut out the old bead, clean and reseal.
Can silicone stain natural stone?
Some standard silicones can leave oily stains on marble, granite and other porous surfaces. For these, use a neutral silicone labelled non-staining for stone. Test on a hidden spot before applying widely.
