PU sealant vs silicone: which one for which joint

The question is usually asked backwards. It is not “which sealant is better” — it is “which joint is this”, and the answer follows from movement, substrate and whether the bead will be painted.

Where silicone wins

• Movement. Silicone tolerates more cyclic movement than standard PU and recovers better after compression.
• UV. Silicone does not need a topcoat in exposed joints. Most PU grades do.
• Temperature. Silicone keeps flexibility from −50 °C to well above any façade temperature.
• Glass-to-glass and glazing. Classic curtain-wall applications.

Where polyurethane wins

• Strength. A PU bead carries load: 2.5 MPa tensile in the body seam sealer, up to 6.5 MPa in the fast windscreen grade.
• Paintability. PU takes a topcoat. Silicone does not — paint will not adhere to it, and silicone contamination spreads.
• Abrasion and tear. PU resists mechanical damage better, which matters on floors, trailers and anywhere a boot or a pallet hits the bead.
• Cost per metre. PU is generally cheaper per litre at the same performance class.

The paintability trap

This is where most of the expensive mistakes happen. A silicone bead in a body shop does not just fail to take paint — the silicone migrates. It contaminates sanding dust, rags and the spray booth, and the next panel gets fish-eyes.

If a joint will be painted, the sealant must be PU (or STP). If a silicone bead has already been there, the surface needs mechanical removal plus an activator before any PU goes on.

Movement, in numbers

A construction joint that opens 5 mm on a 20 mm width is asking for 25% movement. KA340A is specified at 615% elongation with 75% elastic recovery — the recovery figure matters as much as the elongation, because a sealant that stretches but does not come back accumulates damage.

Silicone will also handle that movement. What it will not do is carry load or take paint.

The joint-by-joint answer

Joint	Answer	Why
Bonded windscreen	PU	Structural load, paint, crash performance
Body seam under topcoat	PU	Sandable, paintable, 600% elongation
Curtain wall glazing	Silicone	UV, movement, no paint
Concrete joint, painted surroundings	PU	Paintability and movement
Exposed roof seam on a trailer	STP or silicone	UV stability without a topcoat
Bathroom / sanitary	Silicone	Mould resistance, movement, no paint

Silicone contamination is a process problem, not a product one

A silicone bead does not need to touch a panel to affect it. The contamination path is:

bead → rag → glove → sanding dust → spray booth → next panel → fish-eyes

The concentration that causes a paint defect is measured in parts per million, which is why a workshop that has ever used silicone cannot rely on cleaning alone. The remedies are procedural:

• Separate tools and rags for silicone work, marked and stored away from the PU bench.
• No silicone aerosols in a shop that paints; the overspray travels.
• Mechanical removal plus an activator on any surface that has held a silicone bead, followed by a coupon test.

A painter who says “something in the shop is causing fish-eyes” is usually right, and the something is usually a tube of silicone that was opened months ago.

The cost comparison, per metre

	PU seam sealer	Silicone glazing sealant
Cartridge price	Lower	Higher
Primer needed	Sometimes	Usually not
Topcoat needed	Yes, if painted	Not possible
Rework if painted	Strip and reseal	Strip, clean, repaint the panel
Service life in a moving joint	Good	Better

The silicone column looks competitive until the “rework if painted” row, which is not a product cost but a panel cost. That row is why the decision is usually made by paintability rather than by price.

What this means for buyers

1. Decide paintability before chemistry — it eliminates half the catalogue.
2. Do not use elongation alone as a proxy for movement capacity; check recovery.
3. Keep silicone out of a body shop entirely, including aerosols and lubricants.
4. For mixed substrates without paint, STP is often the better third option.