Neutral Cure vs Acetic Silicone: Why It Matters on Aluminium

Tell them apart by smell

Open a cartridge of acetic cure silicone and it smells sharply of vinegar. That is not a coincidence or an additive — it is acetic acid, released as the sealant cures.

Neutral cure silicone releases alcohol or oxime compounds instead, and smells much milder.

That smell is the whole difference, and it is the one thing you can check without reading a label.

Why acetic acid is a problem here

Acetic acid attacks aluminium. On an anodised surface it degrades the oxide layer that gives the finish its durability. On powder coating it works at the edges and any point where the coating is thin, and once it reaches the metal it corrodes it.

The result is not immediate. It shows up months later as dulling, white bloom or staining along the sealant line, and by then the only fix is to strip it back and start again.

The same applies to any coated metal, to marble and to natural stone, all of which are acid-sensitive.

So on aluminium windows, doors and glazing, use neutral cure. There is no case where acetic is the better choice on a coated aluminium section.

Where acetic is fine

Acetic cure silicone cures faster, bonds very well to glass and glazed ceramic, and costs less. On glass-to-glass joints, glazed tile and sanitary ware with no coated metal involved, it does a good job.

It is a reasonable product used in the right place. The problem is that it is the default on many shelves, and aluminium fabrication is exactly where the default is wrong.

Getting a seal that lasts

Most failed sealant joints we hear about failed for one of two reasons, and neither is the sealant.

Silicone will not bond to old silicone. If you are resealing, the previous bead has to come off completely, back to clean aluminium or glass. A fresh bead over a degraded one is stuck to something that is already letting go.

Surfaces have to be genuinely clean and dry. Dust, oil from handling, or residual moisture in a joint all prevent adhesion. A wipe with a suitable solvent before applying is not an optional refinement.

Cut the nozzle to the joint width rather than the largest opening that will fit, tool the bead once and firmly, and leave it alone. Repeated tooling drags partially skinned silicone away from the edges.

PU foam is a different product

Polyurethane foam and silicone are not interchangeable, and people substitute them because both come in a cartridge.

PU foam expands to fill a cavity — the gap behind a frame, the space between a frame and rough masonry. It insulates and it fills. It is not a weather seal and it degrades in sunlight, so it must be covered.

Silicone seals a joint against water and air. It does not fill a cavity of any size.

A well-sealed window frequently uses both: foam behind the frame to fill and insulate, silicone at the visible perimeter to seal. Using either alone for both jobs is why some frames still draw draughts after being sealed.