Cured silicone is a release surface. Nothing sticks to it, including new silicone. That single fact decides the whole re-seal: the bead you put in is only as good as the substrate you got back to, and a film of the old one left in the corner of the joint is not a shortcut, it is the failure you will be called back to.
Quick answers
Can you put new silicone over old silicone?
No. New silicone bonds to glass and to metal. It does not bond to cured silicone. A bead laid over the old one holds for a season on the friction of the joint and then peels away in one piece, usually taking the look of a proper repair with it.
How do you remove old silicone from glass and aluminium?
Mechanically, in a strip. Cut both bond lines with a sharp blade, free one end, and pull the whole bead out as a length. Then take the residue off with a blade held almost flat, and finish with a solvent wipe.
How much residue is acceptable?
None. There is no thickness of old silicone that a new bead will bond through. If you can feel it with a fingernail, it is still there.
Do I need a silicone remover?
Usually not on glass and metal, where a blade does the work. Where one helps, use a gel product made for the job and read what it leaves behind, because several household tricks leave an oily film that is itself a release agent.
The strip, in order
- Cut both bond lines. A fresh blade, flat against each face of the joint, top to bottom. You are separating the bead from the two surfaces, not slicing the bead.
- Free one end and pull. A steady, even pull brings the whole length out and leaves far less behind than tearing it in pieces. Warmth helps a brittle old bead come away in one go rather than shattering.
- Get the old backer rod out too. It is compressed, dirty and the wrong size for the joint as it now is. It costs almost nothing to replace and it sets the depth of everything that follows.
- Take the residue back. Blade almost flat on glass, and gently on coated aluminium where a steep angle will score the finish. Plastic scrapers for anything anodised or painted.
- Solvent wipe, two cloths. One to wet and lift, a second clean and dry to take it off before it flashes. One cloth simply spreads the film along the joint.
- Do not touch the bond faces after that. Skin oil is a release agent, and a fingerprint is exactly the size of the spot that will fail first.
What to keep off the joint
- Anything oily. Penetrating sprays and lubricants soften old sealant beautifully and leave a film no wipe fully removes. They turn a one day job into a bond that never takes.
- Household solvents you cannot identify. Some attack the gasket, the coating, or the edge seal of an insulated unit, and the damage shows up months later as a fogged pane.
- Strong solvents near an insulated glass edge seal. The perimeter of a sealed unit is chemistry too, and it is not yours.
- Wire wheels and aggressive abrasives. They take the anodising off along with the silicone, and then nothing bonds because there is nothing sound left to bond to.
Do not rebuild the joint you just took out
An old joint is usually being re-sealed because it failed, and a bead put back in the same shape fails the same way. Once the joint is bare, look at it:
- Is there a rod in it? A bead stuck to the back of the joint as well as both sides cannot stretch, and it tears itself apart. That is three-sided adhesion and it is the most common cause of the callback. The five ways joints fail.
- Is the shape right? Roughly twice as wide as it is deep, hourglassed at the face. What a correctly shaped joint uses, in feet per sausage.
- Is the rod the right diameter? It should have to be pushed in, not dropped in. Rod size by joint width.
Test the substrate you actually have now
A stripped joint is not the same surface the original sealant went onto. There may have been a primer, the coating has weathered, and on a retrofit nobody can tell you what was used twenty years ago. Run a bead on an offcut or an inconspicuous length, let it cure fully, and pull it. Where it breaks tells you whether to carry on. The field adhesion test, and how to read the break.
And on anodised or coated aluminium, whatever came out, what goes back in is neutral cure. Acetoxy attacks the finish as it cures and the joint fails at the coating. Why there is only one answer there.
Then give it the time
A re-seal is judged by whether it is still there in five years, and the last hour of the job is the one most often skipped. The bead needs its cure before water, before a wash down, before any load. Skin, through-cure and load are three different days.
