| Where it works on the hair | On and just inside the cuticle. The internal bonds are not touched, so nothing is broken and nothing is reformed. | Into the shaft itself, depositing protein and conditioning agents into the gaps an open cuticle has left. |
| The complaint it answers | Frizz that lifts the moment the weather changes, on hair that behaves the rest of the time. | Hair gone dry, dull, rough and porous — from colour, bleach, daily heat, sun or hard water. |
| The reading that selects it | A wet strand that gives and returns, plus density and texture — thick, coarse hair holds it longest. | A water spray that disappears at once on hair that then dries too fast, and a strand that snaps rather than giving. |
| How much heat it puts through the shaft | It is set with heat and the sealing pass runs hot, so tolerance of heat — not the look you want — sets eligibility. | Considerably less. It is the one chosen where the hair is delicate precisely because the sealing treatments run far hotter. |
| On bleached or heavily lightened hair | Declined where the ends are porous and chalky. Sealing fragile hair traps the damage rather than repairing it. | Often exactly the right treatment — until the lightening is heavy and repeated. Then there is too little structure left to fill: a fortnight of looking well, and the same breakage returns. Bond repair and a cut come first instead. |
| What happens to curl and wave | Softened, organised and better defined; it returns fully as the treatment fades. It makes straightening easy — it does not perform it. | Loosened and calmed, never removed. Hair meant to be straight after one sitting is being sold the wrong treatment. |
| On fine hair | It can flatten it if the wrong formulation is used or it is applied near the root — a lighter product, less of it, kept well away from the scalp. | It adds weight, so on hair that is fine and already coated with product it reads flat rather than smooth. |
| How it ends | Fades gradually across the length. No regrowth line, no correction appointment — you simply decide whether to repeat it. | The same: it grows out with no demarcation and no deadline, leaving the way it arrived. |
| What decides how long it holds | Wash frequency, water hardness, sulphates, salt and chlorine — plus your own hair, since thick coarse hair holds a keratin seal as long as anything kept here. | How porous the hair was to begin with, and then the same routine list. Stated as weeks to a few months because that is the honest span. |
| What is recommended instead when it is refused | A repair plan where the strand will not hold heat, and a cut where the ends are the problem — elasticity has to come back first. | Conditioning work where the hair is basically sound and only a little dry; a cut and bond repair where it is breaking rather than dry. |