# PU sealant vs silicone: which one for which joint

> PU and silicone compared on adhesion, movement, paintability and UV — with the four joints where the wrong choice usually ends in a comeback.

Updated: 2026-11-10 · Source: https://karmindadhesive.com/resources/pu-sealant-vs-silicone-sealant/

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.