Most failures in polyurea spray application are not chemistry failures. They are preparation, primer or equipment failures. Pinholes, blisters, soft spots and poor adhesion nearly always trace back to the substrate or to the spray rig.
This guide walks through the sequence that an applicator or site engineer should check. All the numbers come from Krypton Chemical technical data sheets (TDS), mainly Polyurea Rayston, a pure polyurea that "can only be applied by hot mechanical spraying equipment".
Step 1: confirm the substrate before polyurea spray application
The Polyurea Rayston TDS sets out clear support requirements. The substrate must be:
- Flat, levelled, even and regular.
- Compact and cohesive, with a pull-off value of at least 1.4 N/mm².
- Free of cracks and fissures, or repaired beforehand.
- Clean and dry, free of dust, oil, organic residue and laitance.
Moisture and temperature limits
| Parameter (Polyurea Rayston TDS) | Requirement |
|---|---|
| Substrate moisture | below 4% |
| Substrate temperature | 10–40 °C |
| Air temperature | 10–40 °C |
| Relative humidity | below 85% |
The TDS notes that higher humidity does not stop polymerisation, but it makes adhesion "increasingly difficult" because of condensation. A practical rule on site: measure the dew point and keep the substrate comfortably above it.
Step 2: mechanical preparation
Concrete must be prepared mechanically, by high-pressure sand or abrasion, to remove the surface layer and obtain an open pore. Sharp irregularities are removed with an abrasive disc. Dust is then removed by brushing or vacuum.
For steel, the substrate must be free of rust, oil and grease, and primed with a PU primer.
Surface preparation standards for steel are published by AMPP (formerly NACE and SSPC). Use them to write the blast grade into the specification instead of "clean steel".
Step 3: choose the primer
Primer choice is the most common place to cut corners. It is also where pinholes start. The Polyurea Rayston TDS troubleshooting table lists "bubbles or open holes in the membrane" and gives the cause: porous substrate, no primer.
| Primer (TDS) | Type | Key data | Use |
|---|---|---|---|
| Rayston Epoxy 100 | 2K epoxy, ~100% solids | Mix 100:47 by weight; pot life 40 min at 25 °C; 80D; for substrates below 4% moisture | General concrete priming and sealing |
| Polyurea Primer | 2K fast-curing polyurea | 300–400 g/m²; pot life 60 min at 25 °C; adhesion to concrete >5 N/mm² | Rigid, porous, dry substrates |
| PU primer | Polyurethane | Steel adhesion 5.3 MPa with Polyurea Rayston | Metal substrates |
The Polyurea Primer TDS adds a few substrate conditions: cohesion at least 1.5 MPa, compressive strength at least 25 MPa, and a 21-day wait on fresh concrete or mortar.
Where underlying moisture is suspected, the Polyurea Rayston TDS recommends two coats of epoxy primer: the first neat, the second with quartz sand broadcast. Spray polyurea only after the primer has fully cured.
Step 4: set up the hot-spray equipment
Pure polyurea gels in seconds (4 s at 25 °C for Polyurea Rayston). The machine does the mixing, so the machine must be right.
Recommended settings from the Polyurea Rayston TDS:
- Component A: 65 °C
- Component B: 75 °C
- Hose: 65 °C
- Spray pressure: at least 140 bar
- Mix ratio: A=1, B=1 by volume (A=1, B=1.17 by weight)
Before loading, precondition both sides to 25–30 °C. Mix component A, including the pigment paste, then recirculate both components while heating. A compressed-air dryer is recommended.
Ratio errors show up as soft or sticky membrane. Krypton's note on 2K mix errors covers the warning signs.
Step 5: thickness and consumption
Polyurea Rayston is applied at 1.5–2.0 kg/m² to obtain 1.5–2 mm. The TDS recommends reaching the full thickness in a single application, and a minimum of 1 kg/m² to avoid open holes.
Check thickness during spraying. On site, track consumption by weight alongside a wet or ultrasonic thickness gauge.
Step 6: cure and return to service
The TDS gives a reference hardness curve (1 mm film, 25 °C, 50% RH):
| Time after spraying | Hardness (Shore A) |
|---|---|
| 5 min | 28 |
| 20 min | 55 |
| 1 h | 70 |
| 24 h | 80 |
| 4 days | 88 |
The membrane is rain-resistant after about 10 minutes under most conditions. That speed is the main reason to choose hot-spray polyurea on roofs and bridge decks with short possession windows.
For bridge work, see the bridge decks application page and the note on ETAG 033 for bridge decks.
Pre-spray checklist
Run through this list before every polyurea spray application, not just on the first day.
- [ ] Pull-off test at least 1.4 N/mm² (membrane) / 1.5 MPa (Polyurea Primer)
- [ ] Moisture below 4%; substrate above dew point
- [ ] Open-pore surface; laitance and dust removed
- [ ] Primer selected, applied and fully cured
- [ ] A 65 °C / B 75 °C / hose 65 °C, at least 140 bar
- [ ] Test patch sprayed and checked for ratio and adhesion
- [ ] Target 1.5–2.0 kg/m², single pass
Key takeaways
- Good polyurea spray application starts with the substrate: at least 1.4 N/mm² pull-off and below 4% moisture.
- Prime porous concrete. Pinholes are usually a primer problem, not a polyurea problem.
- Follow the TDS temperatures and pressure; ratio errors show as soft or sticky film.
- Spray full thickness in one pass and verify it during application.
FAQ
What surface preparation is needed for polyurea?
Mechanical preparation (abrasive blasting or grinding) to an open pore, a sound substrate with adequate pull-off strength, low moisture, and a suitable primer.
Why does sprayed polyurea get pinholes?
Usually because air escapes from a porous, unprimed substrate while the fast-gelling polyurea is curing. An epoxy primer seals the pores.
What temperature should polyurea be sprayed at?
For Polyurea Rayston, the TDS gives 65 °C for component A, 75 °C for component B and 65 °C for the hose, at a minimum of 140 bar.
Can polyurea be applied without a spray machine?
Pure hot-spray polyurea cannot. Where a rig is impractical, a cold-applied product such as Impermax Cold Polyurea Supreme is the intended alternative. See hot-spray vs cold-applied.
Further reading
- Technical notes and data sheets at Krypton Chemical Solutions.
- Full product range at kryptonchemical.com.
About the author
Krypton Chemical, S.L. Marketing Department, Nidal Malkawi
Krypton Chemical has manufactured construction chemicals in Spain since 1999. Technical resources: kryptonchemicalsolutions.com · Products: kryptonchemical.com
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