A scalp is skin with a hair problem on top of it. That changes almost nothing about what an oil does and everything about how you have to get it there — and it means the useful application is before the shampoo, not after.
These get treated as one thing on a shelf and they are three. An oil helps with one of them.
Small white flakes, tight feeling, worse in winter and after heat. This is the one hemp oil actually helps — it is barrier dryness on skin that happens to have hair on it.
Larger yellowish flakes, oily roots, often itchy. Usually seborrhoeic rather than dry, and adding oil is at best neutral. This wants an antifungal shampoo and a pharmacist.
Frequently a reaction to a product rather than a scalp condition. Strip everything back to one plain shampoo for two weeks before adding anything new, including this.
Oil after washing sits on hair and looks greasy. Oil before washing does the work and leaves with the shampoo. This is the whole method.
Dry, not damp — the opposite of the rule for body skin, because wet hair stops the oil reaching the scalp at all.
Section by section with fingertips. The point is the skin. Oil on mid-lengths is what makes this look like a bad idea afterwards.
Fingertips, not nails. The massage matters as much as the oil for a tight, flaky scalp.
Longer is not better and overnight is unnecessary. A shower cap keeps it off the pillow and keeps it warm, which helps.
Shampoo first, water second — putting water on first makes the oil far harder to lift. Two short washes beat one long one.
More often is not more effective and eventually leaves buildup. Give it a month before judging.
Hair is not alive. Nothing applied to the length repairs it — oils coat, smooth the cuticle and reduce the friction that causes breakage, which makes hair look and behave better without changing the fibre. Hemp oil does this about as well as any light oil and worse than some heavier ones.
Where it does earn its place on the length is as a tiny amount on the ends of dry or coarse hair, and as a pre-wash barrier that reduces how much water the hair absorbs and swells during washing. Both are real and neither is dramatic.
No, and anything claiming otherwise is overselling. A comfortable, non-inflamed scalp is a better environment for the hair you already grow, which is a real but modest benefit and not the same claim.
Too much oil, or water before shampoo. Half the amount, and shampoo the dry oiled hair directly before wetting it. Two short washes lift it where one long one does not.
You can, and there is little evidence it helps beyond about an hour. It does reliably ruin a pillowcase, so if you do, put an old one on.
Yes — it is an oil, not a solvent, and pre-wash oiling can slightly reduce how much colour washes out by limiting how far water swells the hair. Leave a couple of days after colouring before starting.