Why Your Gel Polish Won't Cure: Lamp Wavelength vs Photoinitiator
Most "bad gel" complaints are a lamp mismatch, not a bad batch. How photoinitiators and lamp peaks have to line up, why the TPO ban changed the answer, and how to test cure in ten minutes.
Short answer: gel cures when a photoinitiator absorbs light at the wavelength your lamp emits. Change the photoinitiator and the same 30-second cure can stop working. Since TPO was banned in the EU on 1 September 2025, many reformulated gels now rely on photoinitiators that need 365 nm — and a lamp that only pushes 405 nm will leave them tacky no matter how long you cure.
The mechanism in one paragraph
A photoinitiator absorbs photons in a narrow band, splits into radicals, and those radicals start the chain reaction that turns monomers into polymer. If the lamp's emission does not overlap the initiator's absorption, nothing happens — the light passes straight through the film. Nail lamps are not full-spectrum: LED units typically emit a few peaks around 365, 395 and 405 nm, while UV fluorescent tubes emit a broader band. "30 seconds" is a validated result for one formula on one lamp, not a property of gel in general.
| Photoinitiator (typical) | Absorption region | Lamp that suits it | Notes |
|---|---|---|---|
| TPO | approx. 380–420 nm | 395 / 405 nm LED | Prohibited in EU cosmetics since 1 Sep 2025 |
| BAPO and derivatives | approx. 365–405 nm | Broad LED or UV tube | Common TPO replacement; yellows slightly at high load |
| TPO-L | approx. 380–405 nm | 395 / 405 nm LED | Liquid analogue under the same regulatory pressure as TPO |
| ITX / amine systems | approx. 380–420 nm | 405 nm LED | Strong through-cure; odour and yellowing trade-offs |
| 365 nm-only systems | approx. 340–380 nm | 365 nm LED | Common in older UV-tube formulas; undercures on 405 nm-only lamps |
Treat the table as a starting point for the conversation with your supplier, not a formulation spec: real absorption depends on the whole system, and the regions above are typical rather than exact peaks. The regulatory side of the same change is covered in TPO vs TMO.
Six reasons a gel stays tacky
- Lamp mismatch: the formula needs 365 nm and the lamp peaks at 405 nm.
- Lamp ageing: LED output falls over years, and a five-year-old lamp can lose a large share of its intensity without failing visibly.
- Distance and angle: curing at 5 cm instead of 1 cm, or holding the hand at an angle, cuts the delivered dose.
- Film thickness: a thick flood of colour or builder cures on top and stays soft underneath. Thin layers, cure between them.
- Mixed systems: using a base or top coat from a different supplier is the most common cause of a sticky inhibition layer that never hardens.
- Cold or old product: viscosity and initiator dispersion change with storage, so a gel kept in a cold room flows and cures differently.
How to test cure properly
- Cure one coat on a clear tip, then press the surface with a dry tissue — no transfer means the surface is cured.
- Rub with acetone for 30 seconds: if colour lifts instantly, the film is undercured rather than just tacky.
- Do a 24-hour check, because shrinkage and lifting show up the next day, not at the lamp.
- Log lamp model, bulb age and cure time next to the batch number — this is the record that resolves complaints.
What to put in a purchase order
- A cure window with the lamp specification: "LED 30 s at 365 + 405 nm, 48 W" is testable, "cures in 30 s" is not.
- The film thickness range the formula was validated at.
- Whether the system is base, colour and top from one supplier, or whether cross-brand use is expected.
- A cure-test result per batch, or at minimum the retention sample that makes a later dispute resolvable.
The commercial consequence of the TPO ban
European salons spent late 2025 discovering that stock containing TPO could not be sold or used, and brands spent the same period reformulating. If you buy gel for the EU, ask for a written TPO-free statement and a cure validation on a named lamp. If you buy for markets that have not restricted TPO, decide deliberately whether you want one global formula or two — running two formulas is more expensive, but a single formula that undercures on your customers' lamps costs more. For the other failure modes that are not about light, see seven gel polish failures.
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