The outline tells you the cause
The single most useful observation about a recurring mark is its shape, because the shape records the geometry of the contact that made it.
A narrow strip a centimetre or two wide, straight or gently curved, is a seam. A ring or a band around a limb is a seal, a cuff or a strap. A diffuse patch at a crease is a panel folding. A line along the base of the neck is a collar with head movement above it. A pair of symmetrical marks is something worn on both sides, which is usually a harness, a rucksack or a hood.
People spend months treating the skin and never look at the shape. The shape is the information.
The usual offenders
The collar. The neck is the joint that moves most while you are wearing a suit, and the collar is thick, stiff and tight by design. The mark is usually at the front or the sides, and it is worse when you have been looking up for two hours, which describes a lot of water sport.
The hood. Worse than the collar because it is the least ventilated item, sits over the ears, temples and forehead, and is often the last thing rinsed and dried. Marks behind the ears and along the hairline are extremely common and are almost always mechanical rather than anything else.
Wrist and ankle seals. A ring, by definition. These are also the sites most frequently washed, so a mechanical problem sits on top of a chemical one.
Underarm panels. A fold that opens and closes several thousand times a session, in the wettest and warmest part of the suit.
Chest zips and sternum seams. A vertical line down the sternum, aggravated by lying prone, which is why paddling sports produce it and swimming does not.
Boots and bootie tops. A ring at the ankle, plus whatever the boot does to the heel, which is a different friction problem with the same physics.
How to isolate it
This is one of the few genuinely testable things in the whole subject.
Note the shape and the exact location, and photograph it if you can bear to, because the memory of a mark is unreliable and the photograph is not.
Then change one thing. Not three. One. A different collar arrangement, a thin smooth layer under the strap, a boot with the top folded differently, a hood one size different, the harness adjusted, the rucksack loaded differently.
Give it three sessions. If the mark is gone, you have found it. If it is unchanged, put that variable back and change the next one.
This is slow and it is the only method that works, and it beats a year of trying products, because a product cannot address a contact.
| Shape | Almost always | First thing to change |
|---|---|---|
| A narrow strip | A seam over a moving joint | Fit at that joint, or a smooth interposed layer |
| A ring around a limb | A seal, cuff or strap | Tension, and whether it is rinsed |
| A diffuse patch at a crease | A panel folding | Fit, and the length of the session |
| A line at the base of the neck | A collar with head movement above it | Collar arrangement and a smooth interface |
| Symmetrical marks | Something worn on both sides: hood, harness, pack | Load and adjustment on both sides |
| Marks behind the ears and at the hairline | A hood, unrinsed | Rinse the hood and the scalp, then fit |
Ordering framework written by this publication from general skin physiology. It is not a measurement, it is not taken from any study, and no number in it is a reading.
Why fit beats material
People assume the answer to a kit related skin problem is a better material. Usually it is a better fit.
Sliding distance is the variable that most directly determines how much of your outer layer is removed, and sliding distance is set by whether the garment moves with your body or independently of it. A garment that is slightly loose at a joint moves independently, and every movement of that joint becomes a sliding contact.
This is also why kit that fits well when you buy it can start causing marks a year later. Neoprene and elastic lose their recovery, garments stretch, and a suit that has gone slightly baggy at the shoulders is now sliding where it used to move with you.
When to interpose something
Interposing a smooth layer between kit and skin reduces the friction coefficient at that contact, which is a legitimate control and a good one where the load is modest.
The general principles are that it needs to stay put for the whole session, needs to be smooth on the skin side, and needs to not itself become a source of pressure or a trap for sand. A thin smooth layer that stays in place beats a thick soft one that rucks up.
We are describing a function rather than naming anything, because we do not name products. The function is: a low friction interface, held in position, that does not add bulk to a contact that is already loaded.
When a mark stops being mechanical
Marks that follow contact geometry, appear after long sessions and settle over a week are mechanical.
Marks that spread beyond the contact outline, that itch severely, that blister, that weep or crust, that recur in the same place with no contact, or that appear with short exposures are not obviously mechanical, and we are not equipped to say what they are. That is a pharmacist or a GP, and if you suspect a reaction to a material, a finish or a cleaning product, say so, because that changes what happens next.