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Almost always yes. Very fine hair is the one real exception, where a leave-in on its own can be enough.

Damp for most hair, dripping wet for curls. On fully dry hair a leave-in tends to sit on top and look greasy — that's a job for a light spray or a few drops of serum instead.

Less than you'd think, and never near the scalp. Mid-lengths and ends, then add more only if it still feels rough once dry.

Yes, and it's usually one of three things: too much product, the wrong format for your hair weight, or applying it too close to the roots. Same product, different placement, different result.

After washing is the default. A light spray in between is fine on curly or dry hair, but daily cream on fine hair will weigh it down within a few days.

Not quite. A leave-in is about moisture and manageability through the length; a serum is a smaller amount on the ends for shine and heat. Plenty of people use both.