The order your skincare goes on in, and why it is free

A serum that does nothing is usually not a bad serum. It is a good serum applied after something that raised the pH of the skin surface out of the range it works in, or under something that stopped it reaching the skin at all. Order is the cheapest improvement available in skincare, because it costs nothing and you already own the products.

The two rules underneath everything

Rule one: thinnest to thickest. Water based before oil based, oil based before anything occlusive. An occlusive layer, which is what most rich creams and all balms are, is designed to stop things moving through it. Put it on first and the serum behind it is decoration.

Rule two: pH sensitive actives go on clean skin, first. Vitamin C as L-ascorbic acid needs a pH below 3.5 to cross the barrier at all. Exfoliating acids work between 3 and 4. If either goes on after a product that has raised the surface pH towards neutral, it is chemically a different product from the one on the label.

A morning that works

• Cleanse, or just water if the skin is dry and nothing happened overnight.
• Vitamin C serum, on skin that is barely damp rather than wet, and then twenty minutes of doing something else.
• Hydrating layer: a humectant serum with glycerin or hyaluronic acid, applied to damp skin so it has water to hold.
• Moisturiser, matched to the season rather than the skin type, because both change.
• Sunscreen, generously, as the last step, and this is the one that does more than the four above it combined.

An evening that works

• Remove the day properly. Oil or balm first if there is sunscreen or make up, then a gentle second cleanse.
• One active, not two. Retinoid on retinoid nights, exfoliating acid on acid nights, and never the same night.
• On a retinoid night, wait until the skin is fully dry, twenty minutes, before applying. Damp skin increases penetration and irritation together.
• Moisturiser on top, which does not reduce the effect of a retinoid and does reduce the peeling.
• Two or three nights a week with no active at all. Barrier repair happens on the nights off, not the nights on.

The combinations that cancel

Vitamin C and niacinamide in the same layer is mostly an internet myth at the concentrations in finished formulas, but vitamin C and an exfoliating acid together is not: two low pH irritants at once is simply more irritation for no additional benefit. Retinoid with an acid in the same session is the most common cause of the flaking, stinging state that people then treat by buying a soothing serum, which they apply on top of both.

Benzoyl peroxide oxidises retinol and several vitamin C forms on contact, so those genuinely belong at opposite ends of the day. And anything containing a high concentration of alcohol under an occlusive will trap the irritation rather than let it evaporate.

Most irritated skin is not sensitive. It is over-treated, and the treatment for that is a fortnight of doing less.

The twenty minute rule

After an acid, the skin surface takes roughly twenty minutes to return towards its normal pH. Applying anything alkaline into that window shortens the acid exposure, which is the only thing you bought the acid for. The wait is inconvenient and it is the difference between a product working and a product being present.

The same applies in reverse to sunscreen, for a different reason. It needs a continuous film to give the protection on the label, and a film does not form over a layer that is still wet. Two minutes of drying time between moisturiser and sunscreen changes the protection meaningfully.

How to fix a shelf

Take everything off it. Put back one cleanser, one hydrating layer, one moisturiser, one sunscreen and at most two actives. Use that for six weeks, which is the shortest honest window for judging anything on skin. Then add one thing, and only one, and give it another six.

Nearly everybody who does this finds two results: the skin improves, and three of the products they took off were doing the same job as each other.

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