The short answer
Sealant fails when it’s applied over old sealant, onto a damp or dirty surface, or from the outside. Old sealant has to come out entirely — scraped and wire-brushed back to bare metal. The surface has to be dry, which usually means waiting a day after rain. And the bead goes on the inside of the channel where water sits, not on the outside where you can see the drip.
Failure cause
Almost always preparation
Old sealant
Must come out entirely
Apply
Inside the channel
Surface
Bone dry before you start
Which sealant?
Three types, and the difference matters more on metal than most surfaces.
Best general choice
Tripolymer or hybrid
Sold specifically as gutter sealant. Adheres to aluminium and galvanised steel, stays flexible through freeze-thaw, and tolerates being applied in cooler weather. Costs more per tube and lasts considerably longer.
Workable
Butyl rubber
Very good adhesion and long-established. Messier to apply and it never fully cures hard, which is why it stays flexible. Fine for seams, awkward around outlets.
Avoid
General-purpose silicone
Bathroom silicone adheres poorly to aluminium and worse to anything previously sealed. It’s the most common thing in a failed gutter joint, and once it’s on, nothing else will stick until it’s all removed.
Why the inside and not the outside
The instinct is to seal where you can see water coming out. But water sits on the inside of the channel and finds its way through the joint from there — sealing the outside means the joint stays wet on the inside and the water simply works around your bead.
Applied inside, the sealant sits under standing water and blocks the path at its source. It’s also invisible from the ground, which is a small aesthetic bonus on a job where the alternative is a visible grey smear.
What resealing can’t fix
If the same joint has been resealed twice and leaks again, the problem probably isn’t the seal. A section holding standing water because of poor fall will keep working at any joint in it. Put a level on that section before buying a third tube.
Doing it properly
Six steps, and four of them are preparation.
Wait for dry weather
A dry day after a dry day. Sealant on a damp surface fails, and the inside of a gutter holds moisture longer than you’d think.
Remove all the old sealant
Scrape with a putty knife, then wire-brush back to bare metal. This is most of the work and skipping it is why the last repair failed.
Clean with solvent
Isopropyl alcohol or mineral spirits on a rag. Removes the film the brush leaves behind.
Let it dry completely
Minutes, but do wait. Sealant over solvent doesn’t adhere.
Bead on the inside
Continuous, over the full joint, on the water side. Tool it flat with a gloved finger so water runs over it rather than catching on an edge.
Leave it to cure before rain
Check the tube — usually a day. Rain on uncured sealant undoes the whole job.
Why the last repair failed
Where this stops being a DIY job
Any joint above single storey. Reaching into a channel to scrape and seal, with both hands occupied, is worse on a ladder than cleaning is.
A joint that has failed three times. That’s a fall or a sag problem holding standing water, and no amount of sealant addresses it.
Rust holes or splits. Sealant over a corroded section buys a season. That length needs replacing.
What a proper repair includes
Prep
Strips back to bare metal
Anyone applying over the old bead is repeating the last failure.
Side
Seals from inside
The water side, not the visible side.
Checks
Puts a level on it
A joint failing repeatedly is a fall problem wearing a sealant costume.
Common questions
What’s the best sealant for gutters?
A tripolymer or hybrid product sold specifically for gutters. It adheres to aluminium, stays flexible through freeze-thaw, and tolerates cool-weather application.
Can I use silicone on gutters?
General-purpose silicone adheres poorly to aluminium and prevents anything else sticking afterwards. It’s the most common cause of a joint that won’t hold a repair.
Should I seal a gutter from the inside or outside?
Inside. Water sits on the inside of the channel, so sealing the outside leaves the joint wet and the water simply works around the bead.
Why does my gutter keep leaking at the same joint?
If it’s failed repeatedly, the section is probably holding standing water because of poor fall. Put a level on it rather than buying more sealant.
Same joint again, or a run holding water?
Stripping back to bare metal, sealing from inside and waiting for genuinely dry weather will fix almost any leaking joint on a ground-floor run. A joint that keeps failing is telling you about the fall instead.
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