Which sealant sticks to uPVC, and why does it keep peeling off?
uPVC is smooth, non-porous, full of migrating plasticiser and moves several millimetres with temperature — four separate reasons an ordinary bead does not last.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 4 min
- Safety
- caution
Short answer
Use a hybrid MS polymer or a neutral-cure silicone stated as suitable for uPVC, and clean the surface with a uPVC-safe cleaner rather than a solvent that crazes it. The joint moves a lot with temperature, so it needs a low modulus product with a good movement rating and a proper width-to-depth ratio — most failures here are movement, not adhesion.
A large white uPVC frame in a masonry opening is one of the hardest joints in a house. The plastic expands and contracts several millimetres across its length between a cold night and a sunny afternoon, the masonry barely moves at all, and the sealant has to absorb the whole difference while stuck to a smooth surface that actively resists adhesion.
Safety
Step by step
- Work out whether the failure is movement or adhesion.A bead that has split down its length is movement or three-sided adhesion. A bead that has peeled cleanly off the plastic in a strip is adhesion. A bead that has peeled off the masonry side is usually a dusty or friable substrate. Each needs a different fix.
- Choose a hybrid MS polymer or a compatible neutral-cure silicone.Hybrids bond to uPVC well, tolerate damp, resist ultraviolet and take paint. Neutral-cure silicone also works where the tube states uPVC compatibility. Acetic silicone is a poor choice here and acrylic frame sealant will not survive the movement outdoors.
- Clean with a uPVC-safe cleaner.uPVC develops a chalky oxidised surface layer outdoors, and the sealant sticks to that rather than to the plastic. A proprietary uPVC cleaner removes it. Do not use acetone, cellulose thinners or aggressive solvents, which craze the surface permanently.
- Abrade only if the manufacturer says to.Some sealant systems specify a light abrasion or a primer on uPVC; most do not, and sanding a visible white frame leaves marks. Read the sealant's own instructions rather than applying general adhesive practice.
- Get the joint geometry right, because this joint moves.Backer rod to give two-sided adhesion and a width-to-depth ratio of about two to one. A frame joint stuck to three faces will split within a year regardless of the product.
- Check the masonry side is sound.Old render, soft mortar and flaking paint all fail with the sealant attached. Rake out and repair a friable edge, or the new bead will lift on that side however good the plastic side is.
- Choose a low modulus product for a render or old-mortar reveal.A stiff sealant pulls the surface off a weak substrate as it stretches. On render, soft brick or old mortar, low modulus is what keeps the render on the wall.
- Seal in reasonable conditions and allow the full cure.Above about five degrees, on a dry surface, with no rain forecast for several hours. An external bead applied in the wrong conditions can look fine and be letting water into the wall all winter.
Tips
- The internal and external beads around a window do different jobs: the external one keeps weather out, the internal one is usually about draughts and appearance. They can reasonably be different products.
- uPVC frames have drainage slots in the outer frame. Never seal over them — they are how water that gets into the frame gets back out, and blocking them causes leaks inside.
- A joint that has been sealed several times may have layers of old sealant in it. Get all of it out; each layer is a bond breaker for the next.
- White sealant yellows outdoors over time and some grades much faster than others. A grade with good ultraviolet stability is worth having on a south-facing elevation.
- If the frame itself moves when you press it, the fixings are the problem and no sealant will hide it.
If it doesn't work
The bead peels off the plastic in one clean strip
Cause: Chalky oxidised surface, or an incompatible sealant — Fix: Clean the uPVC with a proprietary uPVC cleaner, dry it, and reseal with a hybrid or a stated-compatible neutral-cure silicone.
The bead splits down the middle each winter
Cause: Movement exceeding the sealant's capability, usually with three-sided adhesion — Fix: Rake it out, fit backer rod, and reseal with a low modulus product with a movement capability of twenty per cent or better at a two to one width-to-depth ratio.
The bead has pulled the render off with it
Cause: A high modulus sealant on a weak substrate — Fix: Repair the render, let it cure, and use a low modulus sealant so it stretches without pulling on the surface.
The joint is far too wide in places
Cause: A frame fitted into an oversized opening — Fix: Fill the bulk with low-expansion frame foam or a packer, trim back, then seal the remaining joint at a sensible width. Do not try to fill a very wide gap with sealant alone.
Questions people ask
Will silicone stick to uPVC?
Neutral-cure silicone stated as suitable for uPVC will, on a properly cleaned surface. The commonest reasons it does not are the chalky oxidised layer on weathered plastic and an acetic-cure product.
Can I paint sealant around a uPVC window?
Silicone, no. A hybrid MS polymer generally takes paint, and it is the product to choose if the joint sits in painted work. Test with your actual paint first.
Why does the sealant round my windows crack every year?
Almost always movement. uPVC expands and contracts substantially with temperature, so the joint needs a low modulus sealant with real movement capability, backer rod, and a width-to-depth ratio of about two to one.
What should I clean uPVC with before sealing?
A cleaner sold for uPVC. Avoid acetone, cellulose thinners and abrasive cleaners, all of which damage or craze the surface permanently.