Short answer
Look at where it failed before you reseal. If the sealant peeled cleanly off one side, the bond failed: dirty, damp or incompatible surface. If it split through the middle, the joint moved more than the bead could take. If the surface came away with the sealant, the substrate was the weak point.
Running a new bead over a failed one hides the problem and repeats it. The old sealant tells you what went wrong, so read it before you cut it out.
Look before you cut
Photograph the joint. Note how old it is, which side failed, and whether the failure is in one spot, at regular intervals or along the whole run. Failures that line up with panel joints or one sunny elevation usually mean movement. Failures in one patch usually mean something went wrong on the day it was applied.
When you remove a sample, check both faces: is there sealant left on the surface, a clean surface, or bits of surface stuck to the sealant?
Peeled off one side: the bond failed
The sealant came away cleanly from one surface. Common causes:
- Dust, oil, soap film or old silicone left on the surface
- A damp surface when the sealant went on
- The wrong sealant for the surface, such as an acetoxy silicone on natural stone or uPVC
- Tooling liquid smeared into the bond line
- More movement than the bond could take
The fix: cut it all out, clean back to bare, dry surfaces and use a sealant that bonds to both. Neutral-cure vs acetoxy explains which surfaces each cure type suits.
Split down the middle: the joint moved too much
The sealant held on to both sides but tore through the bead. The joint is moving more than the bead can stretch. Usually it's too narrow, too thin, bonded to the back as well as the sides, or it was moved before it had cured.
A stronger sealant won't fix this. The joint needs to be wider, the bead needs the right depth, or it needs backer rod. See joint design basics.
Surface came away: the substrate failed
If paint, render, grout or concrete is stuck to the back of the sealant, the bond was stronger than the surface. Resealing onto the same surface will fail the same way. Remove the weak layer, repair it, and seal onto sound material.
Bubbles, soft spots and stains
- Bubbles: a punctured backer rod, a damp substrate, or air trapped while gunning.
- Still soft after a week: too deep, an expired cartridge, or sealant laid over contamination. See cure times.
- Stains beside the joint: often plasticiser from an incompatible sealant or gasket migrating into stone or render. Test on a hidden spot first next time. See adhesion and compatibility testing.
Mould and leaks
Black spots on a bathroom bead that is still stuck down are surface mould on soap film. Black under the bead, or behind a lifted edge, means water is getting in. Fix the leak or the lifted joint, then reseal. Why bathroom silicone turns black covers this in detail.
If water is coming through a wall, window or roof, the sealant may be the symptom rather than the cause. Find the water path first.
Where it failed tells you what to fix
Clean peel from one side
Remove it all, clean and dry the surface, and choose a sealant that bonds to that material.
Tear through the middle
Widen the joint, correct the depth or add backer rod before resealing.
Surface stuck to the sealant
Repair the weak paint, render or concrete first, then seal onto sound material.
Black under the bead
Find and stop the water getting behind it, dry the joint out, then reseal with an anti-fungal silicone.
Send us a photo of the failure
A close-up of the failed joint and a note of the two surfaces is usually enough for us to tell you why and what to use instead.
Send a photo
