industrial · process
Primerless STP claims, tested by substrate
Updated 2026-11-10
STP sealants are usually sold as primerless. The claim is broadly true on clean substrates and quietly false on the ones that cause failures: oily galvanised steel, previously sealed surfaces, and anything with a release agent.
What primerless means chemically
The silane groups in an STP formulation can react with hydroxyl groups on a mineral or metal oxide surface, so the adhesive bonds directly without an intermediate primer film. That works when the surface actually presents those groups — clean glass, aluminium, cured paint, concrete.
It does not work through oil, dust, mould release or silicone residue, because the sealant never reaches the substrate.
The substrate matrix
| Substrate | Primerless? | Note |
|---|---|---|
| Clean aluminium | Yes | Degrease first |
| Galvanised steel, new | Usually | Oil and passivation vary by coil |
| Galvanised steel, weathered | Usually | Oxide is a weak layer; abrade |
| Powder coat | Usually | Test; some coatings are release-heavy |
| Cured 2K paint | Usually | Test if freshly polished |
| PVC / ABS | Usually | Solvent wipe only |
| GRP | Yes after abrade | Wax release agent must go |
| Previously silicone-sealed | No | Remove residue, activator, test |
| Concrete | Yes if dry | Dust and laitance must go |
The ten-minute test
Bond a 100 mm bead to an offcut, press it to a 3–5 mm thickness, cure 24 hours at room temperature, then peel at 90°. Cohesive failure means the bond is stronger than the sealant. Clean interfacial failure means the surface won.
Run it for each substrate batch, not each joint. It costs one cartridge and one day.
Where an activator earns its place
- Previously sealed surfaces, even after mechanical removal.
- Oily or unknown galvanised stock.
- Any substrate where the coupon peels cleanly.
- Cold surfaces where adhesion development is slow.
What the silane bonds to
An STP bonds through silane groups that hydrolyse and condense with hydroxyl groups on the substrate surface. Mineral substrates — glass, concrete, aluminium oxide, cured paint — present those groups naturally. Organic surfaces and contaminated metals do not.
That gives a simple prediction rule:
| Surface chemistry | Primerless adhesion |
|---|---|
| Oxide or mineral, clean and dry | Good |
| Organic coating, fully cured | Usually good |
| Organic coating with release agents | Poor |
| Metal with oil, grease or soap | Poor |
| Metal with a weak oxide layer | Variable — the oxide fails before the bond |
| Silicone-contaminated anything | Very poor |
The third row is the one that catches production lines: a powder coat that was formulated with a silicone additive releases that additive slowly, and the bond improves after abrasion and a solvent wipe. The sixth row is the one that catches repair work: silicone residue survives mechanical removal in microscopic quantities and transfers by touch.
Ageing changes the surface, in both directions
Weathered aluminium grows a thicker oxide, which presents more hydroxyl groups but is also mechanically weaker. A fresh coupon on weathered metal can pass and a production part still fail, because the failure is in the oxide layer rather than at the interface.
The test that catches it is a peel with the oxide attached to the adhesive — a “clean” peel on weathered metal is the warning sign, not the reassuring result.
What this means for buyers
- Treat “primerless” as a hypothesis to test, not a specification.
- Run the coupon on every substrate batch that changes supplier.
- Keep an activator on site for the exceptions.
- Ask the supplier which substrates the claim was tested on, not just whether it is primerless.