Summer does not ruin leather straps. Three specific things do, in this order: salt water, sweat, and the heat that dries the leather after it has been wet. Knowing which of the three you are facing changes entirely what belongs on your wrist.
Every September straps come back to me stiff, with the edge opening and a whitish bloom on the lining. It is almost always the same story: a swim in the sea without taking the watch off, and then the sun finishing the job.
Why salt water is the worst damage
Salt does not evaporate. Water does. When a strap soaked in seawater dries, the salt stays inside the fibres as crystals. Those crystals are abrasive, and every time the strap flexes — thousands of times a day — they cut the fibre from within.
The result does not show immediately. It shows a few weeks later, when the leather turns stiff and cracks at the fold near the case. By then there is no recovery: the structure of the leather is gone, and no treatment rebuilds it.
If it happens, the only thing that genuinely helps is rinsing in fresh water straight away, before it dries, then drying away from sunlight and any heat source. It does not save the strap, but it buys it a few more months.
Sweat, which is a different problem
Sweat is not water: it is water with salts and oils, slightly acidic. It attacks the leather from the lining side, the one against your wrist, and it does so slowly and constantly.
A strap worn daily through summer with no rotation reaches September with a darkened lining and a smell that does not come out. It is why an untreated calf lining holds up worse than a rubber lining, which is waterproof and can be washed.
The real defence is not a product: it is rotation. Two straps alternated day by day last more than twice as long as one worn continuously, because the leather gets time to dry completely between wears. It is the most ordinary advice I give, and the one that works best.
What holds and what does not
Rubber and technical textiles are the only serious answer for the sea. They rinse, they dry, they do not change. If the watch goes in the water, leather is not in the discussion.
Among leathers, shell cordovan behaves best with humidity — its structure is denser than ordinary leather and absorbs less. But it is not waterproof: it takes water marks that remain visible, and under direct sun it can go dull. It handles sweat; it does not handle the sea.
Vegetable-tanned leather is the most delicate of all with water. It darkens in patches and those patches never go back — in fact they become part of the patina over time, and some clients like that. But it should be a deliberate choice, not an accident you suffer.
Alligator and exotic leathers I advise against in summer, and not because of price: the scales have edges that trap salt and dirt in the gaps, and cleaning them without damage is laboratory work.
The sun, the damage nobody accounts for
Ultraviolet light fades aniline-dyed leathers, which means nearly all the good ones. The damage is gradual and one-sided: the exposed face of the strap lightens, the one under the wrist does not. After one summer you can see the difference between the two faces.
On dark colours it is barely noticeable. On blues, greens and reds it shows a great deal. There is no remedy beyond not leaving the watch in the sun when you take it off — poolside, on a dashboard, on a café table.
What I would wear in July
For the city and for work, a thin leather with a rubber lining. Thickness matters more than material: a thin strap makes you sweat less because it lets the wrist breathe, and in summer two millimetres are felt.
For the sea and the pool, rubber or textile and nothing else. The leather strap comes off before, not after.
For evenings and occasions, whatever you like: leather in summer is no problem as long as it does not meet water and prolonged sweat. An alligator at dinner runs no risk at all.
What I do not know
On water-repellent treatments applied to leather during tanning I have no settled opinion. They are relatively recent, they promise a lot, and I have not yet seen enough treated straps reach three or four years to say whether they hold. When I have watched them age, I will say so.
The short version
Salt water is irreversible and should be avoided, not managed. Sweat is fought with rotation between two straps, not with products. Sun fades bright colours, so the watch does not get left in the sun when you take it off. And for the sea there is one answer only: rubber, and the leather stays at home.
If you spend the summer between the city and the coast and want a single strap that handles both, the honest answer is that it does not exist. You need two, and one of them costs very little.